# fix rigid command Accelerator Variants: *rigid/omp* # fix rigid/nve command Accelerator Variants: *rigid/nve/omp* # fix rigid/nvt command Accelerator Variants: *rigid/nvt/omp* # fix rigid/npt command Accelerator Variants: *rigid/npt/omp* # fix rigid/nph command Accelerator Variants: *rigid/nph/omp* # fix rigid/small command Accelerator Variants: *rigid/small/omp* # fix rigid/nve/small command # fix rigid/nvt/small command # fix rigid/npt/small command # fix rigid/nph/small command ## Syntax fix ID group-ID style bodystyle args keyword values ... - ID, group-ID are documented in [fix](fix) command - style = *rigid* or *rigid/nve* or *rigid/nvt* or *rigid/npt* or *rigid/nph* or *rigid/small* or *rigid/nve/small* or *rigid/nvt/small* or *rigid/npt/small* or *rigid/nph/small* - bodystyle = *single* or *molecule* or *group* *single* args = none *molecule* args = none *custom* args = *i_propname* or *v_varname* i_propname = a custom integer vector defined via fix property/atom v_varname = an atom-style or atomfile-style variable *group* args = N groupID1 groupID2 ... N = # of groups groupID1, groupID2, ... = list of N group IDs - zero or more keyword/value pairs may be appended - keyword = *langevin* or *reinit* or *temp* or *iso* or *aniso* or *x* or *y* or *z* or *couple* or *tparam* or *pchain* or *dilate* or *force* or *torque* or *infile* or *gravity* *langevin* values = Tstart Tstop Tperiod seed Tstart,Tstop = desired temperature at start/stop of run (temperature units) Tdamp = temperature damping parameter (time units) seed = random number seed to use for white noise (positive integer) *reinit* value = *yes* or *no* *temp* values = Tstart Tstop Tdamp Tstart,Tstop = desired temperature at start/stop of run (temperature units) Tdamp = temperature damping parameter (time units) *iso* or *aniso* values = Pstart Pstop Pdamp Pstart,Pstop = scalar external pressure at start/end of run (pressure units) Pdamp = pressure damping parameter (time units) *x* or *y* or *z* values = Pstart Pstop Pdamp Pstart,Pstop = external stress tensor component at start/end of run (pressure units) Pdamp = stress damping parameter (time units) *couple* value = *none* or *xyz* or *xy* or *yz* or *xz* *tparam* values = Tchain Titer Torder Tchain = length of Nose/Hoover thermostat chain Titer = number of thermostat iterations performed Torder = 3 or 5 = Yoshida-Suzuki integration parameters *pchain* values = Pchain Pchain = length of the Nose/Hoover thermostat chain coupled with the barostat *dilate* value = dilate-group-ID dilate-group-ID = only dilate atoms in this group due to barostat volume changes *force* values = M xflag yflag zflag M = which rigid body from 1-Nbody (see asterisk form below) xflag,yflag,zflag = off/on if component of center-of-mass force is active *torque* values = M xflag yflag zflag M = which rigid body from 1-Nbody (see asterisk form below) xflag,yflag,zflag = off/on if component of center-of-mass torque is active *infile* filename filename = file with per-body values of mass, center-of-mass, moments of inertia *gravity* values = gravity-ID gravity-ID = ID of fix gravity command to add gravitational forces ## Examples ``` LAMMPS fix 1 clump rigid single reinit yes fix 1 clump rigid/small molecule fix 1 clump rigid single force 1 off off on langevin 1.0 1.0 1.0 428984 fix 1 polychains rigid/nvt molecule temp 1.0 1.0 5.0 reinit no fix 1 polychains rigid molecule force 1*5 off off off force 6*10 off off on fix 1 polychains rigid/small molecule langevin 1.0 1.0 1.0 428984 fix 2 fluid rigid group 3 clump1 clump2 clump3 torque * off off off fix 1 rods rigid/npt molecule temp 300.0 300.0 100.0 iso 0.5 0.5 10.0 fix 1 particles rigid/npt molecule temp 1.0 1.0 5.0 x 0.5 0.5 1.0 z 0.5 0.5 1.0 couple xz fix 1 water rigid/nph molecule iso 0.5 0.5 1.0 fix 1 particles rigid/npt/small molecule temp 1.0 1.0 1.0 iso 0.5 0.5 1.0 variable bodyid atom 1.0*gmask(clump1)+2.0*gmask(clump2)+3.0*gmask(clump3) fix 1 clump rigid custom v_bodyid variable bodyid atomfile bodies.txt fix 1 clump rigid custom v_bodyid fix 0 all property/atom i_bodyid read_restart data.rigid fix 0 NULL Bodies fix 1 clump rigid/small custom i_bodyid ``` ## Description Treat one or more sets of atoms as independent rigid bodies. This means that each timestep the total force and torque on each rigid body is computed as the sum of the forces and torques on its constituent particles. The coordinates, velocities, and orientations of the atoms in each body are then updated so that the body moves and rotates as a single entity. This is implemented by creating internal data structures for each rigid body and performing time integration on these data structures. Positions, velocities, and orientations of the constituent particles are regenerated from the rigid body data structures in every time step. This restricts which operations and fixes can be applied to rigid bodies. See below for a detailed discussion. Examples of large rigid bodies are a colloidal particle, or portions of a biomolecule such as a protein. Example of small rigid bodies are patchy nanoparticles, such as those modeled in [this paper](Zhang1) by Sharon Glotzer\'s group, clumps of granular particles, lipid molecules consisting of one or more point dipoles connected to other spheroids or ellipsoids, irregular particles built from line segments (2d) or triangles (3d), and coarse-grain models of nano or colloidal particles consisting of a small number of constituent particles. Note that the [fix shake](fix_shake) command can also be used to rigidify small molecules of 2, 3, or 4 atoms, e.g. water molecules. That fix treats the constituent atoms as point masses. These fixes also update the positions and velocities of the atoms in each rigid body via time integration, in the NVE, NVT, NPT, or NPH ensemble, as described below. There are two main variants of this fix, fix rigid and fix rigid/small. The NVE/NVT/NPT/NHT versions belong to one of the two variants, as their style names indicate. :::: note ::: title Note ::: Not all of the *bodystyle* options and keyword/value options are available for both the *rigid* and *rigid/small* variants. See details below. :::: The *rigid* styles are typically the best choice for a system with a small number of large rigid bodies, each of which can extend across the domain of many processors. It operates by creating a single global list of rigid bodies, which all processors contribute to. MPI_Allreduce operations are performed each timestep to sum the contributions from each processor to the force and torque on all the bodies. This operation will not scale well in parallel if large numbers of rigid bodies are simulated. The *rigid/small* styles are typically best for a system with a large number of small rigid bodies. Each body is assigned to the atom closest to the geometrical center of the body. The fix operates using local lists of rigid bodies owned by each processor and information is exchanged and summed via local communication between neighboring processors when ghost atom info is accumulated. :::: note ::: title Note ::: To use the *rigid/small* styles the ghost atom cutoff must be large enough to span the distance between the atom that owns the body and every other atom in the body. This distance value is printed out when the rigid bodies are defined. If the [pair_style](pair_style) cutoff plus neighbor skin does not span this distance, then you should use the [comm_modify cutoff](comm_modify) command with a setting epsilon larger than the distance. :::: Which of the two variants is faster for a particular problem is hard to predict. The best way to decide is to perform a short test run. Both variants should give identical numerical answers for short runs. Long runs should give statistically similar results, but round-off differences may accumulate to produce divergent trajectories. :::: note ::: title Note ::: You should not update the atoms in rigid bodies via other time-integration fixes (e.g. [fix nve](fix_nve), [fix nvt](fix_nh), [fix npt](fix_nh), [fix move](fix_move)), or you will have conflicting updates to positions and velocities resulting in unphysical behavior in most cases. When performing a hybrid simulation with some atoms in rigid bodies, and some not, a separate time integration fix like [fix nve](fix_nve) or [fix nvt](fix_nh) should be used for the non-rigid particles. :::: :::: note ::: title Note ::: These fixes are overkill if you simply want to hold a collection of atoms stationary or have them move with a constant velocity. A simpler way to hold atoms stationary is to not include those atoms in your time integration fix. E.g. use \"fix 1 mobile nve\" instead of \"fix 1 all nve\", where \"mobile\" is the group of atoms that you want to move. You can move atoms with a constant velocity by assigning them an initial velocity (via the [velocity](velocity) command), setting the force on them to 0.0 (via the [fix setforce](fix_setforce) command), and integrating them as usual (e.g. via the [fix nve](fix_nve) command). :::: :::: warning ::: title Warning ::: The aggregate properties of each rigid body are calculated at the start of a simulation run and are maintained in internal data structures. The properties include the position and velocity of the center-of-mass of the body, its moments of inertia, and its angular momentum. This is done using the properties of the constituent atoms of the body at that point in time (or see the *infile* keyword option). Thereafter, changing these properties of individual atoms in the body will have no effect on a rigid body\'s dynamics, unless they effect any computation of per-atom forces or torques. If the keyword *reinit* is set to *yes* (the default), the rigid body data structures will be recreated at the beginning of each *run* command; if the keyword *reinit* is set to *no*, the rigid body data structures will be built only at the very first *run* command and maintained for as long as the rigid fix is defined. For example, you might think you could displace the atoms in a body or add a large velocity to each atom in a body to make it move in a desired direction before a second run is performed, using the [set](set) or [displace_atoms](displace_atoms) or [velocity](velocity) commands. But these commands will not affect the internal attributes of the body unless *reinit* is set to *yes*. With *reinit* set to *no* (or using the *infile* option, which implies *reinit* *no*) the position and velocity of individual atoms in the body will be reset when time integration starts again. :::: ------------------------------------------------------------------------ Each rigid body must have two or more atoms. An atom can belong to at most one rigid body. Which atoms are in which bodies can be defined via several options. :::: note ::: title Note ::: With the *rigid/small* styles, which require that *bodystyle* be specified as *molecule* or *custom*, you can define a system that has no rigid bodies initially. This is useful when you are using the *mol* keyword in conjunction with another fix that is adding rigid bodies on-the-fly as molecules, such as [fix deposit](fix_deposit) or [fix pour](fix_pour). :::: For bodystyle *single* the entire fix group of atoms is treated as one rigid body. This option is only allowed for the *rigid* styles. For bodystyle *molecule*, atoms are grouped into rigid bodies by their respective molecule IDs: each set of atoms in the fix group with the same molecule ID is treated as a different rigid body. This option is allowed for both the *rigid* and *rigid/small* styles. Note that atoms with a molecule ID = 0 will be treated as a single rigid body. For a system with atomic solvent (typically this is atoms with molecule ID = 0) surrounding rigid bodies, this may not be what you want. Thus you should be careful to use a fix group that only includes atoms you want to be part of rigid bodies. Bodystyle *custom* is similar to bodystyle *molecule* except that it is more flexible in using other per-atom properties to define the sets of atoms that form rigid bodies. A custom per-atom integer vector defined by the [fix property/atom](fix_property_atom) command can be used. Or an [atom-style or atomfile-style variable](variable) can be used; the floating-point value produced by the variable is rounded to an integer. As with bodystyle *molecule*, each set of atoms in the fix groups with the same integer value is treated as a different rigid body. Since fix property/atom custom vectors and atom-style variables produce values for all atoms, you should be careful to use a fix group that only includes atoms you want to be part of rigid bodies. :::: note ::: title Note ::: To compute the initial center-of-mass position and other properties of each rigid body, the image flags for each atom in the body are used to \"unwrap\" the atom coordinates. Thus you must ensure that these image flags are consistent so that the unwrapping creates a valid rigid body (one where the atoms are close together), particularly if the atoms in a single rigid body straddle a periodic boundary. This means the input data file or restart file must define the image flags for each atom consistently or that you have used the [set](set) command to specify them correctly. If a dimension is non-periodic then the image flag of each atom must be 0 in that dimension, else an error is generated. :::: The *force* and *torque* keywords discussed next are only allowed for the *rigid* styles. By default, each rigid body is acted on by other atoms which induce an external force and torque on its center of mass, causing it to translate and rotate. Components of the external center-of-mass force and torque can be turned off by the *force* and *torque* keywords. This may be useful if you wish a body to rotate but not translate, or vice versa, or if you wish it to rotate or translate continuously unaffected by interactions with other particles. Note that if you expect a rigid body not to move or rotate by using these keywords, you must ensure its initial center-of-mass translational or angular velocity is 0.0. Otherwise the initial translational or angular momentum the body has will persist. An xflag, yflag, or zflag set to *off* means turn off the component of force of torque in that dimension. A setting of *on* means turn on the component, which is the default. Which rigid body(s) the settings apply to is determined by the first argument of the *force* and *torque* keywords. It can be an integer M from 1 to Nbody, where Nbody is the number of rigid bodies defined. A wild-card asterisk can be used in place of, or in conjunction with, the M argument to set the flags for multiple rigid bodies. This takes the form \"\*\" or \"\*n\" or \"n\*\" or \"m\*n\". If N = the number of rigid bodies, then an asterisk with no numeric values means all bodies from 1 to N. A leading asterisk means all bodies from 1 to n (inclusive). A trailing asterisk means all bodies from n to N (inclusive). A middle asterisk means all types from m to n (inclusive). Note that you can use the *force* or *torque* keywords as many times as you like. If a particular rigid body has its component flags set multiple times, the settings from the final keyword are used. :::: note ::: title Note ::: For computational efficiency, you may wish to turn off pairwise and bond interactions within each rigid body, as they no longer contribute to the motion. The [neigh_modify exclude](neigh_modify) and [delete_bonds](delete_bonds) commands are used to do this. If the rigid bodies have strongly overlapping atoms, you may need to turn off these interactions to avoid numerical problems due to large equal/opposite intra-body forces swamping the contribution of small inter-body forces. :::: For computational efficiency, you should typically define one fix rigid or fix rigid/small command which includes all the desired rigid bodies. LAMMPS will allow multiple rigid fixes to be defined, but it is more expensive. ------------------------------------------------------------------------ The constituent particles within a rigid body can be point particles (the default in LAMMPS) or finite-size particles, such as spheres or ellipsoids or line segments or triangles. See the [atom_style sphere and ellipsoid and line and tri](atom_style) commands for more details on these kinds of particles. Finite-size particles contribute differently to the moment of inertia of a rigid body than do point particles. Finite-size particles can also experience torque (e.g. due to [frictional granular interactions](pair_gran)) and have an orientation. These contributions are accounted for by these fixes. Forces between particles within a body do not contribute to the external force or torque on the body. Thus for computational efficiency, you may wish to turn off pairwise and bond interactions between particles within each rigid body. The [neigh_modify exclude](neigh_modify) and [delete_bonds](delete_bonds) commands are used to do this. For finite-size particles this also means the particles can be highly overlapped when creating the rigid body. ------------------------------------------------------------------------ The *rigid*, *rigid/nve*, *rigid/small*, and *rigid/small/nve* styles perform constant NVE time integration. They are referred to below as the 4 NVE rigid styles. The only difference is that the *rigid* and *rigid/small* styles use an integration technique based on Richardson iterations. The *rigid/nve* and *rigid/small/nve* styles uses the methods described in the paper by [Miller](Miller3), which are thought to provide better energy conservation than an iterative approach. The *rigid/nvt* and *rigid/nvt/small* styles performs constant NVT integration using a Nose/Hoover thermostat with chains as described originally in [(Hoover)](Hoover) and [(Martyna)](Martyna2), which thermostats both the translational and rotational degrees of freedom of the rigid bodies. They are referred to below as the 2 NVT rigid styles. The rigid-body algorithm used by *rigid/nvt* is described in the paper by [Kamberaj](Kamberaj). The *rigid/npt*, *rigid/nph*, *rigid/npt/small*, and *rigid/nph/small* styles perform constant NPT or NPH integration using a Nose/Hoover barostat with chains. They are referred to below as the 4 NPT and NPH rigid styles. For the NPT case, the same Nose/Hoover thermostat is also used as with *rigid/nvt* and *rigid/nvt/small*. The barostat parameters are specified using one or more of the *iso*, *aniso*, *x*, *y*, *z* and *couple* keywords. These keywords give you the ability to specify 3 diagonal components of the external stress tensor, and to couple these components together so that the dimensions they represent are varied together during a constant-pressure simulation. The effects of these keywords are similar to those defined in [fix npt/nph](fix_nh) :::: note ::: title Note ::: Currently the *rigid/npt*, *rigid/nph*, *rigid/npt/small*, and *rigid/nph/small* styles do not support triclinic (non-orthogonal) boxes. :::: The target pressures for each of the 6 components of the stress tensor can be specified independently via the *x*, *y*, *z* keywords, which correspond to the 3 simulation box dimensions. For each component, the external pressure or tensor component at each timestep is a ramped value during the run from *Pstart* to *Pstop*. If a target pressure is specified for a component, then the corresponding box dimension will change during a simulation. For example, if the *y* keyword is used, the y-box length will change. A box dimension will not change if that component is not specified, although you have the option to change that dimension via the [fix deform](fix_deform) command. For all barostat keywords, the *Pdamp* parameter operates like the *Tdamp* parameter, determining the time scale on which pressure is relaxed. For example, a value of 10.0 means to relax the pressure in a timespan of (roughly) 10 time units (e.g. $\tau$ or fs or ps - see the [units](units) command). Regardless of what atoms are in the fix group (the only atoms which are time integrated), a global pressure or stress tensor is computed for all atoms. Similarly, when the size of the simulation box is changed, all atoms are re-scaled to new positions, unless the keyword *dilate* is specified with a *dilate-group-ID* for a group that represents a subset of the atoms. This can be useful, for example, to leave the coordinates of atoms in a solid substrate unchanged and controlling the pressure of a surrounding fluid. Another example is a system consisting of rigid bodies and point particles where the barostat is only coupled with the rigid bodies. This option should be used with care, since it can be unphysical to dilate some atoms and not others, because it can introduce large, instantaneous displacements between a pair of atoms (one dilated, one not) that are far from the dilation origin. The *couple* keyword allows two or three of the diagonal components of the pressure tensor to be \"coupled\" together. The value specified with the keyword determines which are coupled. For example, *xz* means the *Pxx* and *Pzz* components of the stress tensor are coupled. *Xyz* means all 3 diagonal components are coupled. Coupling means two things: the instantaneous stress will be computed as an average of the corresponding diagonal components, and the coupled box dimensions will be changed together in lockstep, meaning coupled dimensions will be dilated or contracted by the same percentage every timestep. The *Pstart*, *Pstop*, *Pdamp* parameters for any coupled dimensions must be identical. *Couple xyz* can be used for a 2d simulation; the *z* dimension is simply ignored. The *iso* and *aniso* keywords are simply shortcuts that are equivalent to specifying several other keywords together. The keyword *iso* means couple all 3 diagonal components together when pressure is computed (hydrostatic pressure), and dilate/contract the dimensions together. Using \"iso Pstart Pstop Pdamp\" is the same as specifying these 4 keywords: x Pstart Pstop Pdamp y Pstart Pstop Pdamp z Pstart Pstop Pdamp couple xyz The keyword *aniso* means *x*, *y*, and *z* dimensions are controlled independently using the *Pxx*, *Pyy*, and *Pzz* components of the stress tensor as the driving forces, and the specified scalar external pressure. Using \"aniso Pstart Pstop Pdamp\" is the same as specifying these 4 keywords: x Pstart Pstop Pdamp y Pstart Pstop Pdamp z Pstart Pstop Pdamp couple none ------------------------------------------------------------------------ The keyword/value option pairs are used in the following ways. The *reinit* keyword determines, whether the rigid body properties are re-initialized between run commands. With the option *yes* (the default) this is done, with the option *no* this is not done. Turning off the re-initialization can be helpful to protect rigid bodies against unphysical manipulations between runs or when properties cannot be easily re-computed (e.g. when read from a file). When using the *infile* keyword, the *reinit* option is automatically set to *no*. The *langevin* and *temp* and *tparam* keywords perform thermostatting of the rigid bodies, altering both their translational and rotational degrees of freedom. What is meant by \"temperature\" of a collection of rigid bodies and how it can be monitored via the fix output is discussed below. The *langevin* keyword applies a Langevin thermostat to the constant NVE time integration performed by any of the 4 NVE rigid styles: *rigid*, *rigid/nve*, *rigid/small*, *rigid/small/nve*. It cannot be used with the 2 NVT rigid styles: *rigid/nvt*, *rigid/small/nvt*. The desired temperature at each timestep is a ramped value during the run from *Tstart* to *Tstop*. The *Tdamp* parameter is specified in time units and determines how rapidly the temperature is relaxed. For example, a value of 100.0 means to relax the temperature in a timespan of (roughly) 100 time units ($\tau$ or fs or ps - see the [units](units) command). The random \# *seed* must be a positive integer. The way that Langevin thermostatting operates is explained on the [fix langevin](fix_langevin) doc page. If you wish to simply viscously damp the rotational motion without thermostatting, you can set *Tstart* and *Tstop* to 0.0, which means only the viscous drag term in the Langevin thermostat will be applied. See the discussion on the [fix viscous](fix_viscous) page for details. :::: note ::: title Note ::: When the *langevin* keyword is used with fix rigid versus fix rigid/small, different dynamics will result for parallel runs. This is because of the way random numbers are used in the two cases. The dynamics for the two cases should be statistically similar, but will not be identical, even for a single timestep. :::: The *temp* and *tparam* keywords apply a Nose/Hoover thermostat to the NVT time integration performed by the 2 NVT rigid styles. They cannot be used with the 4 NVE rigid styles. The desired temperature at each timestep is a ramped value during the run from *Tstart* to *Tstop*. The *Tdamp* parameter is specified in time units and determines how rapidly the temperature is relaxed. For example, a value of 100.0 means to relax the temperature in a timespan of (roughly) 100 time units (tau or fs or ps - see the [units](units) command). Nose/Hoover chains are used in conjunction with this thermostat. The *tparam* keyword can optionally be used to change the chain settings used. *Tchain* is the number of thermostats in the Nose Hoover chain. This value, along with *Tdamp* can be varied to dampen undesirable oscillations in temperature that can occur in a simulation. As a rule of thumb, increasing the chain length should lead to smaller oscillations. The keyword *pchain* specifies the number of thermostats in the chain thermostatting the barostat degrees of freedom. :::: note ::: title Note ::: There are alternate ways to thermostat a system of rigid bodies. You can use [fix langevin](fix_langevin) to treat the individual particles in the rigid bodies as effectively immersed in an implicit solvent, e.g. a Brownian dynamics model. For hybrid systems with both rigid bodies and solvent particles, you can thermostat only the solvent particles that surround one or more rigid bodies by appropriate choice of groups in the compute and fix commands for temperature and thermostatting. The solvent interactions with the rigid bodies should then effectively thermostat the rigid body temperature as well without use of the Langevin or Nose/Hoover options associated with the fix rigid commands. :::: ------------------------------------------------------------------------ The *mol* keyword can only be used with the *rigid/small* styles. It must be used when other commands, such as [fix deposit](fix_deposit) or [fix pour](fix_pour), add rigid bodies on-the-fly during a simulation. You specify a *template-ID* previously defined using the [molecule](molecule) command, which reads a file that defines the molecule. You must use the same *template-ID* that the other fix which is adding rigid bodies uses. The coordinates, atom types, atom diameters, center-of-mass, and moments of inertia can be specified in the molecule file. See the [molecule](molecule) command for details. The only settings required to be in this file are the coordinates and types of atoms in the molecule, in which case the molecule command calculates the other quantities itself. Note that these other fixes create new rigid bodies, in addition to those defined initially by this fix via the *bodystyle* setting. Also note that when using the *mol* keyword, extra restart information about all rigid bodies is written out whenever a restart file is written out. See the NOTE in the next section for details. ------------------------------------------------------------------------ The *infile* keyword allows a file of rigid body attributes to be read in from a file, rather then having LAMMPS compute them. There are 5 such attributes: the total mass of the rigid body, its center-of-mass position, its 6 moments of inertia, its center-of-mass velocity, and the 3 image flags of the center-of-mass position. For rigid bodies consisting of point particles or non-overlapping finite-size particles, LAMMPS can compute these values accurately. However, for rigid bodies consisting of finite-size particles which overlap each other, LAMMPS will ignore the overlaps when computing these 4 attributes. The amount of error this induces depends on the amount of overlap. To avoid this issue, the values can be pre-computed (e.g. using Monte Carlo integration). The format of the file is as follows. Note that the file does not have to list attributes for every rigid body integrated by fix rigid. Only bodies which the file specifies will have their computed attributes overridden. The file can contain initial blank lines or comment lines starting with \"#\" which are ignored. The first non-blank, non-comment line should list N = the number of lines to follow. The N successive lines contain the following information: ID1 masstotal xcm ycm zcm ixx iyy izz ixy ixz iyz vxcm vycm vzcm lx ly lz ixcm iycm izcm ID2 masstotal xcm ycm zcm ixx iyy izz ixy ixz iyz vxcm vycm vzcm lx ly lz ixcm iycm izcm ... IDN masstotal xcm ycm zcm ixx iyy izz ixy ixz iyz vxcm vycm vzcm lx ly lz ixcm iycm izcm The rigid body IDs are all positive integers. For the *single* bodystyle, only an ID of 1 can be used. For the *group* bodystyle, IDs from 1 to Ng can be used where Ng is the number of specified groups. For the *molecule* bodystyle, use the molecule ID for the atoms in a specific rigid body as the rigid body ID. The masstotal and center-of-mass coordinates (xcm,ycm,zcm) are self-explanatory. The center-of-mass should be consistent with what is calculated for the position of the rigid body with all its atoms unwrapped by their respective image flags. If this produces a center-of-mass that is outside the simulation box, LAMMPS wraps it back into the box. The 6 moments of inertia (ixx,iyy,izz,ixy,ixz,iyz) should be the values consistent with the current orientation of the rigid body around its center of mass. The values are with respect to the simulation box XYZ axes, not with respect to the principal axes of the rigid body itself. LAMMPS performs the latter calculation internally. The (vxcm,vycm,vzcm) values are the velocity of the center of mass. The (lx,ly,lz) values are the angular momentum of the body. The (vxcm,vycm,vzcm) and (lx,ly,lz) values can simply be set to 0 if you wish the body to have no initial motion. The (ixcm,iycm,izcm) values are the image flags of the center of mass of the body. For periodic dimensions, they specify which image of the simulation box the body is considered to be in. An image of 0 means it is inside the box as defined. A value of 2 means add 2 box lengths to get the true value. A value of -1 means subtract 1 box length to get the true value. LAMMPS updates these flags as the rigid bodies cross periodic boundaries during the simulation. :::: note ::: title Note ::: If you use the *infile* or *mol* keywords and write restart files during a simulation, then each time a restart file is written, the fix also write an auxiliary restart file with the name rfile.rigid, where \"rfile\" is the name of the restart file, e.g. tmp.restart.10000 and tmp.restart.10000.rigid. This auxiliary file is in the same format described above. Thus it can be used in a new input script that restarts the run and re-specifies a rigid fix using an *infile* keyword and the appropriate filename. Note that the auxiliary file will contain one line for every rigid body, even if the original file only listed a subset of the rigid bodies. :::: ------------------------------------------------------------------------ If you use a [temperature compute](compute) with a group that includes particles in rigid bodies, the degrees-of-freedom removed by each rigid body are accounted for in the temperature (and pressure) computation, but only if the temperature group includes all the particles in a particular rigid body. A 3d rigid body has 6 degrees of freedom (3 translational, 3 rotational), except for a collection of point particles lying on a straight line, which has only 5, e.g a dimer. A 2d rigid body has 3 degrees of freedom (2 translational, 1 rotational). :::: note ::: title Note ::: You may wish to explicitly subtract additional degrees-of-freedom if you use the *force* and *torque* keywords to eliminate certain motions of one or more rigid bodies. LAMMPS does not do this automatically. :::: The rigid body contribution to the pressure of the system (virial) is also accounted for by this fix. ------------------------------------------------------------------------ If your simulation is a hybrid model with a mixture of rigid bodies and non-rigid particles (e.g. solvent) there are several ways these rigid fixes can be used in tandem with [fix nve](fix_nve), [fix nvt](fix_nh), [fix npt](fix_nh), and [fix nph](fix_nh). If you wish to perform NVE dynamics (no thermostatting or barostatting), use one of 4 NVE rigid styles to integrate the rigid bodies, and [fix nve](fix_nve) to integrate the non-rigid particles. If you wish to perform NVT dynamics (thermostatting, but no barostatting), you can use one of the 2 NVT rigid styles for the rigid bodies, and any thermostatting fix for the non-rigid particles ([fix nvt](fix_nh), [fix langevin](fix_langevin), [fix temp/berendsen](fix_temp_berendsen)). You can also use one of the 4 NVE rigid styles for the rigid bodies and thermostat them using [fix langevin](fix_langevin) on the group that contains all the particles in the rigid bodies. The net force added by [fix langevin](fix_langevin) to each rigid body effectively thermostats its translational center-of-mass motion. Not sure how well it does at thermostatting its rotational motion. If you wish to perform NPT or NPH dynamics (barostatting), you cannot use both [fix npt](fix_nh) and the NPT or NPH rigid styles. This is because there can only be one fix which monitors the global pressure and changes the simulation box dimensions. So you have 3 choices: 1. Use one of the 4 NPT or NPH styles for the rigid bodies. Use the *dilate* all option so that it will dilate the positions of the non-rigid particles as well. Use [fix nvt](fix_nh) (or any other thermostat) for the non-rigid particles. 2. Use [fix npt](fix_nh) for the group of non-rigid particles. Use the *dilate* all option so that it will dilate the center-of-mass positions of the rigid bodies as well. Use one of the 4 NVE or 2 NVT rigid styles for the rigid bodies. 3. Use [fix press/berendsen](fix_press_berendsen) to compute the pressure and change the box dimensions. Use one of the 4 NVE or 2 NVT rigid styles for the rigid bodies. Use [fix nvt](fix_nh) (or any other thermostat) for the non-rigid particles. In all case, the rigid bodies and non-rigid particles both contribute to the global pressure and the box is scaled the same by any of the barostatting fixes. You could even use the second and third options for a non-hybrid simulation consisting of only rigid bodies, assuming you give [fix npt](fix_nh) an empty group, though it\'s an odd thing to do. The barostatting fixes ([fix npt](fix_nh) and [fix press/berensen](fix_press_berendsen)) will monitor the pressure and change the box dimensions, but not time integrate any particles. The integration of the rigid bodies will be performed by fix rigid/nvt. ------------------------------------------------------------------------ Styles with a *gpu*, *intel*, *kk*, *omp*, or *opt* suffix are functionally the same as the corresponding style without the suffix. They have been optimized to run faster, depending on your available hardware, as discussed on the [Accelerator packages](Speed_packages) page. The accelerated styles take the same arguments and should produce the same results, except for round-off and precision issues. These accelerated styles are part of the GPU, INTEL, KOKKOS, OPENMP, and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the [Build package](Build_package) page for more info. You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the [-suffix command-line switch](Run_options) when you invoke LAMMPS, or you can use the [suffix](suffix) command in your input script. See the [Accelerator packages](Speed_packages) page for more instructions on how to use the accelerated styles effectively. ------------------------------------------------------------------------ ## Restart, fix_modify, output, run start/stop, minimize info No information about the 4 NVE rigid styles is written to [binary restart files](restart). The exception is if the *infile* or *mol* keyword is used, in which case an auxiliary file is written out with rigid body information each time a restart file is written, as explained above for the *infile* keyword. For the 2 NVT rigid styles, the state of the Nose/Hoover thermostat is written to [binary restart files](restart). Ditto for the 4 NPT and NPH rigid styles, and the state of the Nose/Hoover barostat. See the [read_restart](read_restart) command for info on how to re-specify a fix in an input script that reads a restart file, so that the operation of the fix continues in an uninterrupted fashion. The [fix_modify](fix_modify) *temp* and *press* options are supported by the 4 NPT and NPH rigid styles to change the computes used to calculate the instantaneous pressure tensor. Note that the 2 NVT rigid fixes do not use any external compute to compute instantaneous temperature. The [fix_modify](fix_modify) *bodyforces* option is supported by all rigid styles to set whether per-body forces and torques are computed early or late in a timestep, i.e. at the post-force stage or at the final-integrate stage or the timestep, respectively. The cumulative energy change in the system imposed by the 6 NVT, NPT, NPH rigid fixes, via either thermostatting and/or barostatting, is included in the [thermodynamic output](thermo_style) keywords *ecouple* and *econserve*. See the [thermo_style](thermo_style) doc page for details. The 2 NVE rigid fixes compute a global scalar which can be accessed by various [output commands](Howto_output). The scalar value calculated by these fixes is \"intensive\". The scalar is the current temperature of the collection of rigid bodies. This is averaged over all rigid bodies and their translational and rotational degrees of freedom. The translational energy of a rigid body is 1/2 m v\^2, where m = total mass of the body and v = the velocity of its center of mass. The rotational energy of a rigid body is 1/2 I w\^2, where I = the moment of inertia tensor of the body and w = its angular velocity. Degrees of freedom constrained by the *force* and *torque* keywords are removed from this calculation, but only for the *rigid* and *rigid/nve* fixes. The 6 NVT, NPT, NPH rigid fixes compute a global scalar which can be accessed by various [output commands](Howto_output). The scalar is the same cumulative energy change due to these fixes described above. The scalar value calculated by this fix is \"extensive\". The [fix_modify](fix_modify) *virial* option is supported by these fixes to add the contribution due to the added forces on atoms to both the global pressure and per-atom stress of the system via the [compute pressure](compute_pressure) and [compute stress/atom](compute_stress_atom) commands. The former can be accessed by [thermodynamic output](thermo_style). The default setting for this fix is [fix_modify virial yes](fix_modify). All of the *rigid* styles (not the *rigid/small* styles) compute a global array of values which can be accessed by various [output commands](Howto_output). Similar information about the bodies defined by the *rigid/small* styles can be accessed via the [compute rigid/local](compute_rigid_local) command. The number of rows in the array is equal to the number of rigid bodies. The number of columns is 15. Thus for each rigid body, 15 values are stored: the xyz coords of the center of mass (COM), the xyz components of the COM velocity, the xyz components of the force acting on the COM, the xyz components of the torque acting on the COM, and the xyz image flags of the COM. The center of mass (COM) for each body is similar to unwrapped coordinates written to a dump file. It will always be inside (or slightly outside) the simulation box. The image flags have the same meaning as image flags for atom positions (see the \"dump\" command). This means you can calculate the unwrapped COM by applying the image flags to the COM, the same as when unwrapped coordinates are written to a dump file. The force and torque values in the array are not affected by the *force* and *torque* keywords in the fix rigid command; they reflect values before any changes are made by those keywords. The ordering of the rigid bodies (by row in the array) is as follows. For the *single* keyword there is just one rigid body. For the *molecule* keyword, the bodies are ordered by ascending molecule ID. For the *group* keyword, the list of group IDs determines the ordering of bodies. The array values calculated by these fixes are \"intensive\", meaning they are independent of the number of atoms in the simulation. No parameter of these fixes can be used with the *start/stop* keywords of the [run](run) command. These fixes are not invoked during [energy minimization](minimize). ------------------------------------------------------------------------ ## Restrictions These fixes are all part of the RIGID package. It is only enabled if LAMMPS was built with that package. See the [Build package](Build_package) page for more info. Assigning a temperature via the [velocity create](velocity) command to a system with [rigid bodies](fix_rigid) may not have the desired outcome for two reasons. First, the velocity command can be invoked before the rigid-body fix is invoked or initialized and the number of adjusted degrees of freedom (DOFs) is known. Thus it is not possible to compute the target temperature correctly. Second, the assigned velocities may be partially canceled when constraints are first enforced, leading to a different temperature than desired. A workaround for this is to perform a [run 0](run) command, which ensures all DOFs are accounted for properly, and then rescale the temperature to the desired value before performing a simulation. For example: ``` LAMMPS velocity all create 300.0 12345 run 0 # temperature may not be 300K velocity all scale 300.0 # now it should be ``` ## Related commands [delete_bonds](delete_bonds), [neigh_modify](neigh_modify) exclude, [fix shake](fix_shake) ## Default The option defaults are force \* on on on and torque \* on on on, meaning all rigid bodies are acted on by center-of-mass force and torque. Also Tchain = Pchain = 10, Titer = 1, Torder = 3, reinit = yes. ------------------------------------------------------------------------ ::: {#Hoover} **(Hoover)** Hoover, Phys Rev A, 31, 1695 (1985). ::: ::: {#Kamberaj} **(Kamberaj)** Kamberaj, Low, Neal, J Chem Phys, 122, 224114 (2005). ::: ::: {#Martyna2} **(Martyna)** Martyna, Klein, Tuckerman, J Chem Phys, 97, 2635 (1992); Martyna, Tuckerman, Tobias, Klein, Mol Phys, 87, 1117. ::: ::: {#Miller3} **(Miller)** Miller, Eleftheriou, Pattnaik, Ndirango, and Newns, J Chem Phys, 116, 8649 (2002). ::: ::: {#Zhang1} **(Zhang)** Zhang, Glotzer, Nanoletters, 4, 1407-1413 (2004). :::