pair_style style args
lj/charmm/coul/charmm/inter args = inner outer (inner2) (outer2) (coeff_style) (rmix_style) (rsoftcore cutoff_ratio) inner, outer = global switching cutoffs for Lennard Jones (and Coulombic if only 2 args) coeff_style = ab, eskl, es4-4, es1-2, es4k4l, or es1k2l (see below) rmix_style = arith, maxmax, maxmin, minmax, minmin, or mix_es_from_ab (see below)
pair_style lj/charmm/coul/charmm/inter 7.5 8.0 pair_coeff 1 1 0.40 3.6 0 0 0.8 3.6(The new pair_style also supports repulsive mixing rules and "soft-core" repulsion.)
pair_style lj/charmm/coul/charmm/inter 7.5 8.0 pair_style lj/charmm/coul/charmm/inter 7.5 8.0 eskl maxmax pair_style lj/charmm/coul/charmm/inter 7.5 8.0 es4k4l maxmax rsoftcore 1.0 pair_style lj/charmm/coul/charmm/inter 7.5 8.0 es1k2l maxmax rsoftcore 0.8909 pair_coeff 1 1 0.40 3.6 pair_coeff 1 1 0.40 3.6 1.0 -0.5 pair_coeff 1 1 0.40 3.6 1 -1 0 0 0 0 0.8 3.6 1 0
All of the lj/charmm styles compute LJ and Coulombic interactions with an additional switching function S(r) that ramps the energy and force smoothly to zero between an inner and outer cutoff. It is a widely used potential in the CHARMM MD code. See (MacKerell) for a description of the CHARMM force field.
Both the LJ and Coulombic terms require an inner and outer cutoff. They can be the same for both formulas or different depending on whether 2 or 4 arguments are used in the pair_style command. In each case, the inner cutoff distance must be less than the outer cutoff. It it typical to make the difference between the 2 cutoffs about 1.0 Angstrom.
coeff_style meaning: required user coeffs: --------------------------------------------------------------------------- ab LJ(r)= A / r^12 - B / r^6, A,B specified by user eskl LJ(r)= e*[K*(s/r)^12 + L*(s/r)^6] e,s,K,L specified by user es4k4l LJ(r)= 4*e*[K*(s/r)^12 + L*(s/r)^6] e,s,K,L specified by user es1k2l LJ(r)= e*[K*(s/r)^12 + 2*L*(s/r)^6] e,s,K,L specified by user es4-4 LJ(r)= 4*e*[K*(s/r)^12 - L*(s/r)^6] e,s (K,L inferred from sign) es1-2 LJ(r)= e*[K*(s/r)^12 - 2*L*(s/r)^6] e,s (K,L inferred from sign)Here "e" and "s" are shorthand for epsilon (ϵ) and sigma (σ). The last two options allow the user to omit K and L. However users have some limited control over K and L by altering the sign of ϵ and σ:
e>0, s>0 --> K=1, L=-1 (default/typical usage) e>0, s<0 --> K=1, L=0 e<0, s<0 --> K=1, L=1 e<0, s>0 --> K=0, L=1The default coeff style is "es4-4". (This way, the syntax of the pair_coeff commands remains unchanged from the syntax used in "lj/charmm/coul/charmm", and the behavior is the same for positive "e" and "s".) Interactions between unlike atom types are determined by mixing . (See below.)
The following coefficients must be defined for each pair of atoms types via the pair_coeff command as in the examples above, or in the data file or restart files read by the read_data or read_restart commands, or by mixing as described below:
The optional epsilon_14 and sigma_14 parameters customize the interactions between the first and fourth atoms participating in a dihedral interaction (IE. "1-4 interactions"). The optional epsilonDIF and sigmaDIF parameters allow the user to customize the interactions between atoms in different molecules. If these parameters are unspecified, they are set to "epsilon" and "sigma" by default. (However you must specify epsilon_14 and sigma_14 if you wish to specify the epsilonDIF and sigmaDIF parameters. They can not be omitted.)
No pair-specific cutoffs are specified because this CHARMM force field does not allow varying cutoffs for individual atom pairs; all pairs use the global cutoff(s) specified in the pair_style command.If you use the "eskl" "es4k4l" or "es1k2l" styles, then you must also specify
pair_coeff 1 1 0.40 3.6 1 -1 0 0 0 0 0.8 3.6 1 0...the user has turned OFF the attractive part of the interaction between atoms in different molecules (by switching the "L" parameter to zero), while leaving the repulsive steric interaction in place.
Mixing, shift, table, tail correction, restart, rRESPA info:
For atom type pairs I,J and I != J, the epsilon, sigma, epsilon_14, and sigma_14, epsilonDIF, and sigmaDIF coefficients for all of the lj/charmm pair styles can be mixed. The default rule for mixing epsilon and sigma parameters is arithmetic to coincide with the usual settings for the CHARMM force field. See the "pair_modify" command for details.
The rule for mixing interactions between one or two repulsive particles is determined by the optional rmix_style argument. By default rmix_style is set to "maxmax". With this choice, repulsive interactions override attractive ones. The other choices are shown below:
rmix_style meaning: ---------------------------------------------------------- maxmax --> Kij=MAX(Ki,Kj), Lij=MAX(Li,Lj) repulsive interactions "win" maxmin --> Kij=MAX(Ki,Kj), Lij=MIN(Li,Lj) minmax --> Kij=MIN(Ki,Kj), Lij=MAX(Li,Lj) minmin --> Kij=MIN(Ki,Kj), Lij=MIN(Li,Lj) arith --> Kij=0.5*(Ki+Kj), Lij=0.5*(Li+Lj) (if coeff_style not "ab") --> Aij=0.5*(Ai+Aj), Bij=0.5*(Bi+Bj) (if coeff_style is "ab") mix_es_from_ab --> only available when coeff_style is "ab"Generally none of these rmix_style settings effect the mixing rules for epsilon and sigma ("e" and "s"). (They are still determined by the preferences set by the "pair_modify" command.)
None of the lj/charmm pair styles support the pair_modify shift option, since the Lennard-Jones portion of the pair interaction is smoothed to 0.0 at the cutoff.
None of the lj/charmm pair styles support the pair_modify tail option for adding long-range tail corrections to energy and pressure, since the Lennard-Jones portion of the pair interaction is smoothed to 0.0 at the cutoff.
All of the lj/charmm pair styles write their information to binary restart files, so pair_style and pair_coeff commands do not need to be specified in an input script that reads a restart file.This style only supports the pair keyword of run_style respa. See the run_style command for details.
The lj/charmm/coul/charmm/inter style is part of the MOLECULE package. It is only enabled if LAMMPS was built with this package. See the Making LAMMPS section for more info. Note that the MOLECULE and KSPACE packages are installed by default.
(MacKerell) MacKerell, Bashford, Bellott, Dunbrack, Evanseck, Field, Fischer, Gao, Guo, Ha, et al, J Phys Chem, 102, 3586 (1998).