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lammps/doc/pair_lj_soft.txt
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"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
:link(lws,http://lammps.sandia.gov)
:link(ld,Manual.html)
:link(lc,Section_commands.html#comm)
:line
pair_style lj/cut/soft command :h3
pair_style lj/cut/coul/cut/soft command :h3
pair_style lj/cut/coul/long/soft command :h3
pair_style lj/cut/tip4p/long/soft command :h3
pair_style coul/cut/soft command :h3
pair_style coul/long/soft command :h3
pair_style tip4p/long/soft command :h3
[Syntax:]
pair_style style args :pre
style = {lj/cut/soft} or {lj/cut/coul/cut/soft} or {lj/cut/coul/long/soft} or {lj/cut/tip4p/long/soft} or {coul/cut/soft} or {coul/long/soft} or {tip4p/long/soft}
args = list of arguments for a particular style :ul
{lj/cut/soft} args = n alpha_lj cutoff
n, alpha_LJ = parameters of soft-core potential
cutoff = global cutoff for Lennard-Jones interactions (distance units)
{lj/cut/coul/cut/soft} args = n alpha_LJ alpha_C cutoff (cutoff2)
n, alpha_LJ, alpha_C = parameters of soft-core potential
cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
cutoff2 = global cutoff for Coulombic (optional) (distance units)
{lj/cut/coul/long/soft} args = n alpha_LJ alpha_C cutoff
n, alpha_LJ, alpha_C = parameters of the soft-core potential
cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
cutoff2 = global cutoff for Coulombic (optional) (distance units)
{lj/cut/tip4p/long/soft} args = otype htype btype atype qdist n alpha_LJ alpha_C cutoff (cutoff2)
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)
n, alpha_LJ, alpha_C = parameters of the soft-core potential
cutoff = global cutoff for LJ (and Coulombic if only 1 arg) (distance units)
cutoff2 = global cutoff for Coulombic (optional) (distance units)
{coul/cut/soft} args = n alpha_C cutoff
n, alpha_C = parameters of the soft-core potential
cutoff = global cutoff for Coulomb interactions (distance units)
{coul/long/soft} args = n alpha_C cutoff
n, alpha_C = parameters of the soft-core potential
cutoff = global cutoff for Coulomb interactions (distance units)
{tip4p/long/soft} args = otype htype btype atype qdist n alpha_C cutoff
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)
n, alpha_C = parameters of the soft-core potential
cutoff = global cutoff for Coulomb interactions (distance units)
:pre
[Examples:]
pair_style lj/cut/soft 2.0 0.5 9.5
pair_coeff * * 0.28 3.1 1.0
pair_coeff 1 1 0.28 3.1 1.0 9.5 :pre
pair_style lj/cut/coul/cut/soft 2.0 0.5 10.0 9.5
pair_style lj/cut/coul/cut/soft 2.0 0.5 10.0 9.5 9.5
pair_coeff * * 0.28 3.1 1.0
pair_coeff 1 1 0.28 3.1 0.5 10.0
pair_coeff 1 1 0.28 3.1 0.5 10.0 9.5 :pre
pair_style lj/cut/coul/long/soft 2.0 0.5 10.0 9.5
pair_style lj/cut/coul/long/soft 2.0 0.5 10.0 9.5 9.5
pair_coeff * * 0.28 3.1 1.0
pair_coeff 1 1 0.28 3.1 0.0 10.0
pair_coeff 1 1 0.28 3.1 0.0 10.0 9.5 :pre
pair_style lj/cut/tip4p/long/soft 1 2 7 8 2.0 0.5 10.0 0.15 9.8
pair_style lj/cut/tip4p/long/soft 1 2 7 8 2.0 0.5 10.0 0.15 9.8 9.5
pair_coeff * * 0.155 3.1536 1.0
pair_coeff 1 1 0.155 3.1536 1.0 9.5 :pre
pair_style coul/long/soft 1.0 10.0 9.5
pair_coeff * * 1.0
pair_coeff 1 1 1.0 9.5 :pre
pair_style tip4p/long/soft 1 2 7 8 2.0 0.5 10.0 0.15 9.8
pair_coeff * * 1.0
pair_coeff 1 1 1.0 9.5 :pre
[Description:]
The {lj/cut/soft} style and substyles compute the 12/6
Lennard-Jones and Coulomb potential modified by a soft core, in order
to avoid singularities during free energy calculations when sites are
created or anihilated "(Beutler)"_#Beutler,
:c,image(Eqs/pair_lj_soft.jpg)
Coulomb interactions are also damped with a soft core at short
distance,
:c,image(Eqs/pair_coul_soft.jpg)
In the Coulomb part C is an energy-conversion constant, q_i and q_j are
the charges on the 2 atoms, and epsilon is the dielectric constant
which can be set by the "dielectric"_dielectric.html command.
The coefficient lambda is an activation parameter. When lambda = 1 the
pair potentiel is identical to a Lennard-Jones term plus a Coulomb
term. When lambda = 0 the pair interactions are deactivated. The
transition between these two extrema is smoothed by a repulsive soft
core in order to avoid singularities in potential energy and forces
when sites are created or anihilated and can overlap
"(Beutler)"_#Beutler.
The paratemers n, alpha_LJ and alpha_C are set in the
"pair_style"_pair_style.html command, before the cutoffs. Usual
choices for the exponent are n = 2 or n = 1. For the remaining
coefficients alpha_LJ = 0.5 and alpha_C = 10 Angstrom^2 are
appropriate choices.
The {lj/cut/tip4p/long/soft} implements a soft-core version of the
TIP4P water model. The usage of this pair style is documented in the
"pair_lj"_pair_lj.html styles. The soft-core version introduces the
lambda parameter to the list of arguments, after epsilon and sigma in
the "pair_coeff"_pair_coeff.html command. The paratemers n, alpha_LJ
and alpha_C are set in the "pair_style"_pair_style.html command,
before the cutoffs.
The {coul/cut/soft}, {coul/long/soft} and {tip4p/long/soft} substyles
are designed to be combined with other pair potentials via the
"pair_style hybrid/overlay"_pair_hybrid.html command. This is because
they have no repulsive core. Hence, if used by themselves, there will
be no repulsion to keep two oppositely charged particles from
overlapping each other. In this case, if lambda = 1, a singularity may
occur. These substyles are suitable to represent charges embedded in
the Lennard-Jones radius of another site (for example hydrogen atoms
in several water models).
For the {lj/cut/coul/cut/soft} or {lj/cut/coul/long/soft} pair styles,
the following coefficients must be defined for each pair of atoms
types via the "pair_coeff"_pair_coeff.html command as in the examples
above, or in the data file or restart files read by the
"read_data"_read_data.html or "read_restart"_read_restart.html
commands, or by mixing as described below:
epsilon (energy units)
sigma (distance units)
lambda (activation parameter between 0 and 1)
cutoff1 (distance units)
cutoff2 (distance units) :ul
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. You cannot specify 2 cutoffs for style {lj/cut/soft},
since it has no Coulombic terms. For the {coul/cut/soft} and
{coul/long/soft} only lambda and the optional cutoff2 are to be
specified.
:line
[Mixing, shift, tail correction, restart info]:
For atom type pairs I,J and I != J, the epsilon and sigma coefficients
and cutoff distance for this pair style 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.html shift
option for the energy of the Lennard-Jones portion of the pair
interaction.
These pair styles support the "pair_modify"_pair_modify.html tail
option for adding a long-range tail correction to the energy and
pressure for the Lennard-Jones portion of the pair interaction.
These pair styles write information to "binary restart
files"_restart.html, so pair_style and pair_coeff commands do not need
to be specified in an input script that reads a restart file.
:line
[Restrictions:]
To avoid division by zero do not set sigma = 0. When sites do not
interact though the Lennard-Jones term use epsilon = 0 and sigma = 1
for example, or else use the {coul/long/soft} or similar substyle.
[Related commands:]
"pair_coeff"_pair_coeff.html, "fix_adapt"_fix_adapt.html,
"compute_fep"_compute_fep.html
[Default:] none
:line
:link(Beutler)
[(Beutler)] Beutler, Mark, van Schaik, Gerber, van Gunsteren, Chem
Phys Lett, 222, 529 (1994).
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