# fix temp/berendsen command ## Syntax fix ID group-ID temp/berendsen Tstart Tstop Tdamp - ID, group-ID are documented in [fix](fix) command - temp/berendsen = style name of this fix command - Tstart,Tstop = desired temperature at start/end of run Tstart can be a variable (see below) - Tdamp = temperature damping parameter (time units) ## Examples ``` LAMMPS fix 1 all temp/berendsen 300.0 300.0 100.0 ``` ## Description Reset the temperature of a group of atoms by using a Berendsen thermostat [(Berendsen)](Berendsen2), which rescales their velocities every timestep. The thermostat is applied to only the translational degrees of freedom for the particles, which is an important consideration for finite-size particles which have rotational degrees of freedom are being thermostatted with this fix. The translational degrees of freedom can also have a bias velocity removed from them before thermostatting takes place; see the description below. 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). *Tstart* can be specified as an equal-style [variable](variable). In this case, the *Tstop* setting is ignored. If the value is a variable, it should be specified as v_name, where name is the variable name. In this case, the variable will be evaluated each timestep, and its value used to determine the target temperature. :::: note ::: title Note ::: This thermostat will generate an error if the current temperature is zero at the end of a timestep. It cannot rescale a zero temperature. :::: Equal-style variables can specify formulas with various mathematical functions, and include [thermo_style](thermo_style) command keywords for the simulation box parameters and timestep and elapsed time. Thus it is easy to specify a time-dependent temperature. :::: note ::: title Note ::: Unlike the [fix nvt](fix_nh) command which performs Nose/Hoover thermostatting AND time integration, this fix does NOT perform time integration. It only modifies velocities to effect thermostatting. Thus you must use a separate time integration fix, like [fix nve](fix_nve) to actually update the positions of atoms using the modified velocities. Likewise, this fix should not normally be used on atoms that also have their temperature controlled by another fix - e.g. by [fix nvt](fix_nh) or [fix langevin](fix_langevin) commands. :::: See the [Howto thermostat](Howto_thermostat) page for a discussion of different ways to compute temperature and perform thermostatting. This fix computes a temperature each timestep. To do this, the fix creates its own compute of style \"temp\", as if this command had been issued: ``` LAMMPS compute fix-ID_temp group-ID temp ``` See the [compute temp](compute_temp) command for details. Note that the ID of the new compute is the fix-ID + underscore + \"temp\", and the group for the new compute is the same as the fix group. Note that this is NOT the compute used by thermodynamic output (see the [thermo_style](thermo_style) command) with ID = *thermo_temp*. This means you can change the attributes of this fix\'s temperature (e.g. its degrees-of-freedom) via the [compute_modify](compute_modify) command or print this temperature during thermodynamic output via the [thermo_style custom](thermo_style) command using the appropriate compute-ID. It also means that changing attributes of *thermo_temp* will have no effect on this fix. Like other fixes that perform thermostatting, this fix can be used with [compute commands](compute) that remove a \"bias\" from the atom velocities. E.g. to apply the thermostat only to atoms within a spatial [region](region), or to remove the center-of-mass velocity from a group of atoms, or to remove the x-component of velocity from the calculation. This is not done by default, but only if the [fix_modify](fix_modify) command is used to assign a temperature compute to this fix that includes such a bias term. See the doc pages for individual [compute temp commands](compute) to determine which ones include a bias. In this case, the thermostat works in the following manner: bias is removed from each atom, thermostatting is performed on the remaining thermal degrees of freedom, and the bias is added back in. ------------------------------------------------------------------------ ## Restart, fix_modify, output, run start/stop, minimize info This fix writes the cumulative global energy change to [binary restart files](restart). 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 fix continues in an uninterrupted fashion. The [fix_modify](fix_modify) *temp* option is supported by this fix. You can use it to assign a temperature [compute](compute) you have defined to this fix which will be used in its thermostatting procedure, as described above. For consistency, the group used by this fix and by the compute should be the same. The cumulative energy change in the system imposed by this fix is included in the [thermodynamic output](thermo_style) keywords *ecouple* and *econserve*. See the [thermo_style](thermo_style) doc page for details. This fix computes a global scalar which can be accessed by various [output commands](Howto_output). The scalar is the same cumulative energy change due to this fix described in the previous paragraph. The scalar value calculated by this fix is \"extensive\". This fix can ramp its target temperature over multiple runs, using the *start* and *stop* keywords of the [run](run) command. See the [run](run) command for details of how to do this. This fix is not invoked during [energy minimization](minimize). ## Restrictions This fix can be used with dynamic groups as defined by the [group](group) command. Likewise it can be used with groups to which atoms are added or deleted over time, e.g. a deposition simulation. However, the conservation properties of the thermostat and barostat are defined for systems with a static set of atoms. You may observe odd behavior if the atoms in a group vary dramatically over time or the atom count becomes very small. ## Related commands [fix nve](fix_nve), [fix nvt](fix_nh), [fix temp/rescale](fix_temp_rescale), [fix langevin](fix_langevin), [fix_modify](fix_modify), [compute temp](compute_temp), [fix press/berendsen](fix_press_berendsen) ## Default none ------------------------------------------------------------------------ ::: {#Berendsen2} **(Berendsen)** Berendsen, Postma, van Gunsteren, DiNola, Haak, J Chem Phys, 81, 3684 (1984). :::