# pair_style lj/long/coul/long command Accelerator Variants: *lj/long/coul/long/intel*, *lj/long/coul/long/omp*, *lj/long/coul/long/opt* # pair_style lj/long/tip4p/long command Accelerator Variants: *lj/long/tip4p/long/omp* ## Syntax ``` LAMMPS pair_style style args ``` - style = *lj/long/coul/long* or *lj/long/tip4p/long* - args = list of arguments for a particular style *lj/long/coul/long* args = flag_lj flag_coul cutoff (cutoff2) flag_lj = *long* or *cut* or *off* *long* = use Kspace long-range summation for dispersion 1/r\^6 term *cut* = use a cutoff on dispersion 1/r\^6 term *off* = omit disperion 1/r\^6 term entirely flag_coul = *long* or *off* *long* = use Kspace long-range summation for Coulombic 1/r term *off* = omit Coulombic term cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units) cutoff2 = global cutoff for Coulombic (optional) (distance units) *lj/long/tip4p/long* args = flag_lj flag_coul otype htype btype atype qdist cutoff (cutoff2) flag_lj = *long* or *cut* *long* = use Kspace long-range summation for dispersion 1/r\^6 term *cut* = use a cutoff flag_coul = *long* or *off* *long* = use Kspace long-range summation for Coulombic 1/r term *off* = omit Coulombic term otype,htype = atom types for TIP4P O and H btype,atype = bond and angle types for TIP4P waters qdist = distance from O atom to massless charge (distance units) cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units) cutoff2 = global cutoff for Coulombic (optional) (distance units) ## Examples ``` LAMMPS pair_style lj/long/coul/long cut off 2.5 pair_style lj/long/coul/long cut long 2.5 4.0 pair_style lj/long/coul/long long long 2.5 4.0 pair_coeff * * 1 1 pair_coeff 1 1 1 3 4 pair_style lj/long/tip4p/long long long 1 2 7 8 0.15 12.0 pair_style lj/long/tip4p/long long long 1 2 7 8 0.15 12.0 10.0 pair_coeff * * 100.0 3.0 pair_coeff 1 1 100.0 3.5 9.0 ``` ## Description Style *lj/long/coul/long* computes the standard 12/6 Lennard-Jones potential: $$\begin{aligned} E = 4 \epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^6 \right] \qquad r < r_c \\ \end{aligned}$$ with $\epsilon$ and $\sigma$ being the usual Lennard-Jones potential parameters, plus the Coulomb potential, given by: $$E = \frac{C q_i q_j}{\epsilon r} \qquad r < r_c$$ where C is an energy-conversion constant, $q_i$ and $q_j$ are the charges on the 2 atoms, $\epsilon$ is the dielectric constant which can be set by the [dielectric](dielectric) command, and $r_c$ is the cutoff. If one cutoff is specified in the pair_style command, it is used for both the LJ and Coulombic terms. If two cutoffs are specified, they are used as cutoffs for the LJ and Coulombic terms respectively. The purpose of this pair style is to capture long-range interactions resulting from both attractive 1/r\^6 Lennard-Jones and Coulombic 1/r interactions. This is done by use of the *flag_lj* and *flag_coul* settings. The [In \'t Veld](Veld2) paper has more details on when it is appropriate to include long-range 1/r\^6 interactions, using this potential. Style *lj/long/tip4p/long* implements the TIP4P water model of [(Jorgensen)](Jorgensen4), which introduces a massless site located a short distance away from the oxygen atom along the bisector of the HOH angle. The atomic types of the oxygen and hydrogen atoms, the bond and angle types for OH and HOH interactions, and the distance to the massless charge site are specified as pair_style arguments. :::: note ::: title Note ::: For each TIP4P water molecule in your system, the atom IDs for the O and 2 H atoms must be consecutive, with the O atom first. This is to enable LAMMPS to \"find\" the 2 H atoms associated with each O atom. For example, if the atom ID of an O atom in a TIP4P water molecule is 500, then its 2 H atoms must have IDs 501 and 502. :::: See the [Howto tip4p](Howto_tip4p) page for more information on how to use the TIP4P pair style. Note that the neighbor list cutoff for Coulomb interactions is effectively extended by a distance 2\*qdist when using the TIP4P pair style, to account for the offset distance of the fictitious charges on O atoms in water molecules. Thus it is typically best in an efficiency sense to use a LJ cutoff \>= Coulombic cutoff + 2\*qdist, to shrink the size of the neighbor list. This leads to slightly larger cost for the long-range calculation, so you can test the trade-off for your model. If *flag_lj* is set to *long*, no cutoff is used on the LJ 1/r\^6 dispersion term. The long-range portion can be calculated by using the [kspace_style ewald/disp or pppm/disp](kspace_style) commands. The specified LJ cutoff then determines which portion of the LJ interactions are computed directly by the pair potential versus which part is computed in reciprocal space via the Kspace style. If *flag_lj* is set to *cut*, the LJ interactions are simply cutoff, as with [pair_style lj/cut](pair_lj). If *flag_coul* is set to *long*, no cutoff is used on the Coulombic interactions. The long-range portion can calculated by using any of several [kspace_style](kspace_style) command options such as *pppm* or *ewald*. Note that if *flag_lj* is also set to long, then the *ewald/disp* or *pppm/disp* Kspace style needs to be used to perform the long-range calculations for both the LJ and Coulombic interactions. If *flag_coul* is set to *off*, Coulombic interactions are not computed. The following coefficients must be defined for each pair of atoms types via the [pair_coeff](pair_coeff) command as in the examples above, or in the data file or restart files read by the [read_data](read_data) or [read_restart](read_restart) commands, or by mixing as described below: - $\epsilon$ (energy units) - $\sigma$ (distance units) - cutoff1 (distance units) - cutoff2 (distance units) Note that sigma is defined in the LJ formula as the zero-crossing distance for the potential, not as the energy minimum at 2\^(1/6) sigma. The latter 2 coefficients are optional. If not specified, the global LJ and Coulombic cutoffs specified in the pair_style command are used. If only one cutoff is specified, it is used as the cutoff for both LJ and Coulombic interactions for this type pair. If both coefficients are specified, they are used as the LJ and Coulombic cutoffs for this type pair. Note that if you are using *flag_lj* set to *long*, you cannot specify a LJ cutoff for an atom type pair, since only one global LJ cutoff is allowed. Similarly, if you are using *flag_coul* set to *long*, you cannot specify a Coulombic cutoff for an atom type pair, since only one global Coulombic cutoff is allowed. For *lj/long/tip4p/long* only the LJ cutoff can be specified since a Coulombic cutoff cannot be specified for an individual I,J type pair. All type pairs use the same global Coulombic cutoff specified in the pair_style command. ------------------------------------------------------------------------ A version of these styles with a soft core, *lj/cut/soft*, suitable for use in free energy calculations, is part of the FEP package and is documented with the [pair_style \*/soft](pair_fep_soft) styles. The version with soft core is only available if LAMMPS was built with that package. See the [Build package](Build_package) page for more info. ------------------------------------------------------------------------ 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. ------------------------------------------------------------------------ ## Mixing, shift, table, tail correction, restart, rRESPA info For atom type pairs I,J and I != J, the epsilon and sigma coefficients and cutoff distance for all of the lj/long pair styles can be mixed. The default mix value is *geometric*. See the \"pair_modify\" command for details. These pair styles support the [pair_modify](pair_modify) shift option for the energy of the Lennard-Jones portion of the pair interaction, assuming *flag_lj* is *cut*. These pair styles support the [pair_modify](pair_modify) table and table/disp options since they can tabulate the short-range portion of the long-range Coulombic and dispersion interactions. Thes pair styles do not support the [pair_modify](pair_modify) tail option for adding a long-range tail correction to the Lennard-Jones portion of the energy and pressure. These pair styles write their information to [binary restart files](restart), so pair_style and pair_coeff commands do not need to be specified in an input script that reads a restart file. The pair lj/long/coul/long styles support the use of the *inner*, *middle*, and *outer* keywords of the [run_style respa](run_style) command, meaning the pairwise forces can be partitioned by distance at different levels of the rRESPA hierarchy. See the [run_style](run_style) command for details. ------------------------------------------------------------------------ ## Restrictions These styles are part of the KSPACE package. They are only enabled if LAMMPS was built with that package. See the [Build package](Build_package) page for more info. ## Related commands [pair_coeff](pair_coeff) ## Default none ------------------------------------------------------------------------ ::: {#Veld2} **(In \'t Veld)** In \'t Veld, Ismail, Grest, J Chem Phys, 127, 144711 (2007). ::: ::: {#Jorgensen4} **(Jorgensen)** Jorgensen, Chandrasekhar, Madura, Impey, Klein, J Chem Phys, 79, 926 (1983). :::