forked from lijiext/lammps
278 lines
12 KiB
Plaintext
278 lines
12 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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pair_style lj/cut/soft command :h3
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pair_style lj/cut/soft/omp command :h3
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pair_style lj/cut/coul/cut/soft command :h3
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pair_style lj/cut/coul/cut/soft/omp command :h3
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pair_style lj/cut/coul/long/soft command :h3
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pair_style lj/cut/coul/long/soft/omp command :h3
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pair_style lj/cut/tip4p/long/soft command :h3
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pair_style lj/cut/tip4p/long/soft/omp command :h3
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pair_style lj/charmm/coul/long/soft command :h3
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pair_style lj/charmm/coul/long/soft/omp command :h3
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pair_style coul/cut/soft command :h3
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pair_style coul/cut/soft/omp command :h3
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pair_style coul/long/soft command :h3
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pair_style coul/long/soft/omp command :h3
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pair_style tip4p/long/soft command :h3
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pair_style tip4p/long/soft/omp command :h3
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[Syntax:]
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pair_style style args :pre
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style = {lj/cut/soft} or {lj/cut/coul/cut/soft} or {lj/cut/coul/long/soft} or {lj/cut/tip4p/long/soft} or {lj/charmm/coul/long/soft} or {coul/cut/soft} or {coul/long/soft} or {tip4p/long/soft}
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args = list of arguments for a particular style :ul
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{lj/cut/soft} args = n alpha_lj cutoff
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n, alpha_LJ = parameters of soft-core potential
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cutoff = global cutoff for Lennard-Jones interactions (distance units)
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{lj/cut/coul/cut/soft} args = n alpha_LJ alpha_C cutoff (cutoff2)
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n, alpha_LJ, alpha_C = parameters of soft-core potential
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cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
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cutoff2 = global cutoff for Coulombic (optional) (distance units)
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{lj/cut/coul/long/soft} args = n alpha_LJ alpha_C cutoff
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n, alpha_LJ, alpha_C = parameters of the soft-core potential
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cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
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cutoff2 = global cutoff for Coulombic (optional) (distance units)
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{lj/cut/tip4p/long/soft} args = otype htype btype atype qdist n alpha_LJ alpha_C cutoff (cutoff2)
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otype,htype = atom types for TIP4P O and H
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btype,atype = bond and angle types for TIP4P waters
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qdist = distance from O atom to massless charge (distance units)
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n, alpha_LJ, alpha_C = parameters of the soft-core potential
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cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
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cutoff2 = global cutoff for Coulombic (optional) (distance units)
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{lj/charmm/coul/long/soft} args = n alpha_LJ alpha_C inner outer (cutoff)
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n, alpha_LJ, alpha_C = parameters of the soft-core potential
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inner, outer = global switching cutoffs for LJ (and Coulombic if only 5 args)
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cutoff = global cutoff for Coulombic (optional, outer is Coulombic cutoff if only 5 args)
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{coul/cut/soft} args = n alpha_C cutoff
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n, alpha_C = parameters of the soft-core potential
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cutoff = global cutoff for Coulomb interactions (distance units)
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{coul/long/soft} args = n alpha_C cutoff
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n, alpha_C = parameters of the soft-core potential
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cutoff = global cutoff for Coulomb interactions (distance units)
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{tip4p/long/soft} args = otype htype btype atype qdist n alpha_C cutoff
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otype,htype = atom types for TIP4P O and H
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btype,atype = bond and angle types for TIP4P waters
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qdist = distance from O atom to massless charge (distance units)
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n, alpha_C = parameters of the soft-core potential
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cutoff = global cutoff for Coulomb interactions (distance units)
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:pre
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[Examples:]
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pair_style lj/cut/soft 2.0 0.5 9.5
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pair_coeff * * 0.28 3.1 1.0
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pair_coeff 1 1 0.28 3.1 1.0 9.5 :pre
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pair_style lj/cut/coul/cut/soft 2.0 0.5 10.0 9.5
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pair_style lj/cut/coul/cut/soft 2.0 0.5 10.0 9.5 9.5
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pair_coeff * * 0.28 3.1 1.0
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pair_coeff 1 1 0.28 3.1 0.5 10.0
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pair_coeff 1 1 0.28 3.1 0.5 10.0 9.5 :pre
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pair_style lj/cut/coul/long/soft 2.0 0.5 10.0 9.5
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pair_style lj/cut/coul/long/soft 2.0 0.5 10.0 9.5 9.5
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pair_coeff * * 0.28 3.1 1.0
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pair_coeff 1 1 0.28 3.1 0.0 10.0
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pair_coeff 1 1 0.28 3.1 0.0 10.0 9.5 :pre
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pair_style lj/cut/tip4p/long/soft 1 2 7 8 0.15 2.0 0.5 10.0 9.8
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pair_style lj/cut/tip4p/long/soft 1 2 7 8 0.15 2.0 0.5 10.0 9.8 9.5
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pair_coeff * * 0.155 3.1536 1.0
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pair_coeff 1 1 0.155 3.1536 1.0 9.5 :pre
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pair_style lj/charmm/coul/long 2.0 0.5 10.0 8.0 10.0
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pair_style lj/charmm/coul/long 2.0 0.5 10.0 8.0 10.0 9.0
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pair_coeff * * 0.28 3.1 1.0
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pair_coeff 1 1 0.28 3.1 1.0 0.14 3.1 :pre
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pair_style coul/long/soft 1.0 10.0 9.5
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pair_coeff * * 1.0
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pair_coeff 1 1 1.0 9.5 :pre
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pair_style tip4p/long/soft 1 2 7 8 0.15 2.0 0.5 10.0 9.8
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pair_coeff * * 1.0
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pair_coeff 1 1 1.0 9.5 :pre
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[Description:]
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The {lj/cut/soft} style and substyles compute the 12/6 Lennard-Jones
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and Coulomb potential modified by a soft core, in order to avoid
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singularities during free energy calculations when sites are created
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or anihilated "(Beutler)"_#Beutler,
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:c,image(Eqs/pair_lj_soft.jpg)
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Coulomb interactions are also damped with a soft core at short
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distance,
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:c,image(Eqs/pair_coul_soft.jpg)
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In the Coulomb part C is an energy-conversion constant, q_i and q_j
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are the charges on the 2 atoms, and epsilon is the dielectric constant
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which can be set by the "dielectric"_dielectric.html command.
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The coefficient lambda is an activation parameter. When lambda = 1 the
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pair potentiel is identical to a Lennard-Jones term or a Coulomb term
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or a combination of both. When lambda = 0 the interactions are
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deactivated. The transition between these two extrema is smoothed by a
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soft repulsive core in order to avoid singularities in potential
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energy and forces when sites are created or anihilated and can overlap
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"(Beutler)"_#Beutler.
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The paratemers n, alpha_LJ and alpha_C are set in the
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"pair_style"_pair_style.html command, before the cutoffs. Usual
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choices for the exponent are n = 2 or n = 1. For the remaining
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coefficients alpha_LJ = 0.5 and alpha_C = 10 Angstrom^2 are
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appropriate choices. Plots of the LJ and Coulomb terms are shown
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below, for lambda ranging from 1 to 0 every 0.1.
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:c,image(JPG/lj_soft.jpg),image(JPG/coul_soft.jpg)
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For the {lj/cut/coul/cut/soft} or {lj/cut/coul/long/soft} pair styles,
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the following coefficients must be defined for each pair of atoms
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types via the "pair_coeff"_pair_coeff.html command as in the examples
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above, or in the data file or restart files read by the
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"read_data"_read_data.html or "read_restart"_read_restart.html
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commands, or by mixing as described below:
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epsilon (energy units)
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sigma (distance units)
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lambda (activation parameter between 0 and 1)
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cutoff1 (distance units)
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cutoff2 (distance units) :ul
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The latter two coefficients are optional. If not specified, the global
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LJ and Coulombic cutoffs specified in the pair_style command are used.
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If only one cutoff is specified, it is used as the cutoff for both LJ
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and Coulombic interactions for this type pair. If both coefficients
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are specified, they are used as the LJ and Coulombic cutoffs for this
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type pair. You cannot specify 2 cutoffs for style {lj/cut/soft},
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since it has no Coulombic terms. For the {coul/cut/soft} and
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{coul/long/soft} only lambda and the optional cutoff2 are to be
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specified.
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Style {lj/cut/tip4p/long/soft} implements a soft-core version of the
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TIP4P water model. The usage of this pair style is documented in the
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"pair_lj"_pair_lj.html styles. The soft-core version introduces the
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lambda parameter to the list of arguments, after epsilon and sigma in
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the "pair_coeff"_pair_coeff.html command. The paratemers n, alpha_LJ
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and alpha_C are set in the "pair_style"_pair_style.html command,
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before the cutoffs.
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Style {lj/charmm/coul/long/soft} implements a soft-core version of the
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CHARMM version of LJ interactions with an additional switching
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function S(r) that ramps the energy and force smoothly to zero between
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an inner and outer cutoff. The usage of this pair style is documented
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in the "pair_charmm"_pair_charmm.html styles. The soft-core version
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introduces the lambda parameter to the list of arguments, after
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epsilon and sigma in the "pair_coeff"_pair_coeff.html command (and
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before the optional eps14 and sigma14). The paratemers n,
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alpha_LJ and alpha_C are set in the "pair_style"_pair_style.html
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command, before the cutoffs.
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The {coul/cut/soft}, {coul/long/soft} and {tip4p/long/soft} substyles
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are designed to be combined with other pair potentials via the
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"pair_style hybrid/overlay"_pair_hybrid.html command. This is because
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they have no repulsive core. Hence, if used by themselves, there will
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be no repulsion to keep two oppositely charged particles from
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overlapping each other. In this case, if lambda = 1, a singularity may
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occur. These substyles are suitable to represent charges embedded in
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the Lennard-Jones radius of another site (for example hydrogen atoms
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in several water models).
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IMPORTANT NOTES: When using the core-softed Coulomb potentials with
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long-range solvers ({coul/long/soft}, {lj/cut/coul/long/soft}, etc.)
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in a free energy calculation in which sites holding electrostatic
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charges are being created or anihilated (using
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"fix_adapt/fep"_fix_adapt_fep.html and "compute_fep"_compute_fep.html)
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it is important to adapt both the lambda activation parameter (from 0
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to 1, or the reverse) and the value of the charge (from 0 to its final
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value, or the reverse). This ensures that long-range electrostatic
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terms (kspace) are correct. It is not necessary to use core-softed
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Coulomb potentials if the van der Waals site is present during the
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free-energy route, thus avoiding overlap of the charges. Examples are
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provided in the LAMMPS source directory tree, under examples/USER/fep.
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:line
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Styles with a {cuda}, {gpu}, {intel}, {kk}, {omp}, or {opt} suffix are
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functionally the same as the corresponding style without the suffix.
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They have been optimized to run faster, depending on your available
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hardware, as discussed in "Section_accelerate"_Section_accelerate.html
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of the manual. The accelerated styles take the same arguments and
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should produce the same results, except for round-off and precision
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issues.
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These accelerated styles are part of the USER-CUDA, GPU, USER-INTEL,
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KOKKOS, USER-OMP and OPT packages, respectively. They are only
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enabled if LAMMPS was built with those packages. See the "Making
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LAMMPS"_Section_start.html#start_3 section for more info.
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You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the "-suffix command-line
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switch"_Section_start.html#start_7 when you invoke LAMMPS, or you can
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use the "suffix"_suffix.html command in your input script.
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See "Section_accelerate"_Section_accelerate.html of the manual for
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more instructions on how to use the accelerated styles effectively.
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:line
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[Mixing, shift, tail correction, restart info]:
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For atom type pairs I,J and I != J, the epsilon and sigma coefficients
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and cutoff distance for this pair style can be mixed.
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The default mix value is {geometric}. See the "pair_modify" command
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for details.
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These pair styles support the "pair_modify"_pair_modify.html shift
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option for the energy of the Lennard-Jones portion of the pair
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interaction.
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These pair styles support the "pair_modify"_pair_modify.html tail
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option for adding a long-range tail correction to the energy and
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pressure for the Lennard-Jones portion of the pair interaction.
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These pair styles write information to "binary restart
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files"_restart.html, so pair_style and pair_coeff commands do not need
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to be specified in an input script that reads a restart file.
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:line
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[Restrictions:]
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To avoid division by zero do not set sigma = 0; use the lambda
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parameter instead to activate/deactivate interactions, or use
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epsilon = 0 and sigma = 1. Alternatively, when sites do not
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interact though the Lennard-Jones term the {coul/long/soft} or
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similar substyle can be used via the
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"pair_style hybrid/overlay"_pair_hybrid.html command.
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:line
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All of the plain {soft} pair styles are part of the USER-FEP package.
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The {long} styles also requires the KSPACE package to be installed.
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They are only enabled if LAMMPS was built with those packages. See
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the "Making LAMMPS"_Section_start.html#start_3 section for more info.
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[Related commands:]
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"pair_coeff"_pair_coeff.html, "fix_adapt"_fix_adapt.html,
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"fix_adapt/fep"_fix_adapt_fep.html, "compute_fep"_compute_fep.html
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[Default:] none
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:line
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:link(Beutler)
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[(Beutler)] Beutler, Mark, van Schaik, Gerber, van Gunsteren, Chem
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Phys Lett, 222, 529 (1994).
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