lammps/doc/pair_gromacs.txt

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pair_style lj/gromacs command :h3
pair_style lj/gromacs/cuda command :h3
pair_style lj/gromacs/coul/gromacs command :h3
pair_style lj/gromacs/coul/gromacs/cuda command :h3

[Syntax:]

pair_style style args :pre

style = {lj/gromacs} or {lj/gromacs/coul/gromacs}
args = list of arguments for a particular style :ul
  {lj/gromacs} args = inner outer
    inner, outer = global switching cutoffs for Lennard Jones
  {lj/gromacs/coul/gromacs} args = inner outer (inner2) (outer2)
    inner, outer = global switching cutoffs for Lennard Jones (and Coulombic if only 2 args)
    inner2, outer2 = global switching cutoffs for Coulombic (optional) :pre

[Examples:]

pair_style lj/gromacs 9.0 12.0
pair_coeff * * 100.0 2.0
pair_coeff 2 2 100.0 2.0 8.0 10.0 :pre

pair_style lj/gromacs/coul/gromacs 9.0 12.0
pair_style lj/gromacs/coul/gromacs 8.0 10.0 7.0 9.0
pair_coeff * * 100.0 2.0 :pre

[Description:]

The {lj/gromacs} 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 commonly
used potential in the "GROMACS"_http://www.gromacs.org MD code and for
the coarse-grained models of "(Marrink)"_#Marrink.

:c,image(Eqs/pair_gromacs.jpg)

R1 is the inner cutoff; Rc is the outer cutoff.  The coefficients A
and B are computed by LAMMPS to perform the smoothing.  The function
S(r) is actually applied once to each term of the LJ formula and once
to the Coulombic formula, so there are 2 or 3 sets of A,B coefficients
depending on which pair_style is used.  The boundary conditions
applied to the smoothing function are as follows: S(r1) = S'(r1) = 0,
S(rc) = -F(rc), S'(rc) = -F'(rc), where F(r) is the correpsonding term
in the LJ or Coulombic function and a single quote represents a
derivative with respect to r.

The inner and outer cutoff for the LJ and Coulombic terms can be the
same or different depending on whether 2 or 4 arguments are used in
the pair_style command.  The inner LJ cutoff must be > 0, but the
inner Coulombic cutoff can be >= 0.

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)
inner (distance units)
outer (distance units) :ul

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 last 2 coefficients are optional inner and outer cutoffs for style
{lj/gromacs}.  If not specified, the global {inner} and {outer} values
are used.

The last 2 coefficients cannot be used with style
{lj/gromacs/coul/gromacs} because this force field does not allow
varying cutoffs for individual atom pairs; all pairs use the global
cutoff(s) specified in the pair_style command.

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Styles with a {cuda}, {gpu}, 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 in "this section"_Section_accelerate.html of the manual.
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 "user-cuda", "gpu", and "opt"
packages respectively.  They are only enabled if LAMMPS was built with
those packages.  See the "Making LAMMPS"_Section_start.html#2_3
section 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"_Section_start.html#2_6 when you invoke LAMMPS, or you can use
the "suffix"_suffix.html command in your input script.

See "this section"_Section_accelerate.html of the manual for more
instructions on how to use the accelerated styles effectively.

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[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/cut pair styles can be mixed.
The default mix value is {geometric}.  See the "pair_modify" command
for details.

None of the GROMACS pair styles support the
"pair_modify"_pair_modify.html shift option, since the Lennard-Jones
portion of the pair interaction is already smoothed to 0.0 at the
cutoff.

The "pair_modify"_pair_modify.html table option is not relevant
for this pair style.

None of the GROMACS pair styles support the
"pair_modify"_pair_modify.html tail option for adding long-range tail
corrections to energy and pressure, since there are no corrections for
a potential that goes to 0.0 at the cutoff.

All of the GROMACS pair styles write their 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.

All of the GROMACS pair styles can only be used via the {pair}
keyword of the "run_style respa"_run_style.html command.  They do not
support the {inner}, {middle}, {outer} keywords.

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[Restrictions:] none

[Related commands:]

"pair_coeff"_pair_coeff.html

[Default:] none

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:link(Marrink)
[(Marrink)] Marrink, de Vries, Mark, J Phys Chem B, 108, 750-760 (2004).