lammps/doc/pair_charmm.txt

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:link(lc,Section_commands.html#comm)

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pair_style lj/charmm/coul/charmm command :h3
pair_style lj/charmm/coul/charmm/implicit command :h3
pair_style lj/charmm/coul/long command :h3
pair_style lj/charmm/coul/long/opt command :h3

[Syntax:]

pair_style style args :pre

style = {lj/charmm/coul/charmm} or {lj/charmm/coul/charmm/implicit} or {lj/charmm/coul/long} or {lj/charmm/coul/long/opt}
args = list of arguments for a particular style :ul
  {lj/charmm/coul/charmm} 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)
  {lj/charmm/coul/charmm/implicit} args = inner outer (inner2) (outer2)
    inner, outer = global switching cutoffs for LJ (and Coulombic if only 2 args)
    inner2, outer2 = global switching cutoffs for Coulombic (optional)
  {lj/charmm/coul/long} args = inner outer (cutoff)
    inner, outer = global switching cutoffs for LJ (and Coulombic if only 2 args)
    cutoff = global cutoff for Coulombic (optional, outer is Coulombic cutoff if only 2 args) :pre

[Examples:]

pair_style lj/charmm/coul/charmm 8.0 10.0
pair_style lj/charmm/coul/charmm 8.0 10.0 7.0 9.0
pair_coeff * * 100.0 2.0
pair_coeff 1 1 100.0 2.0 150.0 3.5 :pre

pair_style lj/charmm/coul/charmm/implicit 8.0 10.0
pair_style lj/charmm/coul/charmm/implicit 8.0 10.0 7.0 9.0
pair_coeff * * 100.0 2.0
pair_coeff 1 1 100.0 2.0 150.0 3.5 :pre

pair_style lj/charmm/coul/long 8.0 10.0
pair_style lj/charmm/coul/long/opt 8.0 10.0
pair_style lj/charmm/coul/long 8.0 10.0 9.0
pair_coeff * * 100.0 2.0
pair_coeff 1 1 100.0 2.0 150.0 3.5 :pre

[Description:]

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 cuoff.  It is a widely
used potential in the "CHARMM"_http://www.scripps.edu/brooks MD code.
See "(MacKerell)"_#MacKerell for a description of the CHARMM force
field.

:c,image(Eqs/pair_charmm.jpg)

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.

Style {lj/charmm/coul/charmm/implicit} computes the same formulas as
style {lj/charmm/coul/charmm} except that an additional 1/r term is
included in the Coulombic formula.  The Coulombic energy thus varies
as 1/r^2.  This is effectively a distance-dependent dielectric term
which is a simple model for an implicit solvent with additional
screening.  It is designed for use in a simulation of an unsolvated
biomolecule (no explicit water molecules).

Style {lj/charmm/coul/long} computes the same formulas as style
{lj/charmm/coul/charmm} except that an additional damping factor is
applied to the Coulombic term, as in the discussion for pair style
{lj/cut/coul/long}.  Only one Coulombic cutoff is specified for
{lj/charmm/coul/long}; if only 2 arguments are used in the pair_style
command, then the outer LJ cutoff is used as the single Coulombic
cutoff.

Style {lj/charmm/coul/long/opt} is an optimized version of style
{lj/charmm/coul/long} that should give identical answers.  Depending
on system size and the processor you are running on, it may be 5-25%
faster (for the pairwise portion of the run time).

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:

epsilon (energy units)
sigma (distance units)
epsilon_14 (energy units)
sigma_14 (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 latter 2 coefficients are optional.  If they are specified, they
are used in the LJ formula between 2 atoms of these types which are
also first and fourth atoms in any dihedral.  No 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 the pair_coeff command is not used to define coefficients for a
particular I != J type pair, the mixing rule for epsilon and sigma for
all CHARMM potentials is to use the {arithmetic} formulas documented
by the "pair_modify"_pair_modify.html command.  The "pair_modify
mix"_pair_modify.html setting is thus ignored for CHARMM potentials.

[Restrictions:]

The {lj/charmm/coul/charmm} and {lj/charmm/coul/charmm/implicit}
styles are part of the "molecule" package.  The {lj/charmm/coul/long}
style is part of the "kspace" package.  The {lj/charmm/coul/long/opt}
style is part of the "opt" package and also requires the "kspace"
package.  They are only enabled if LAMMPS was built with those
package(s).  See the "Making LAMMPS"_Section_start.html#2_3 section
for more info.

On some 64-bit machines, compiling with -O3 appears to break the
Coulombic tabling option used by the {lj/charmm/coul/long} style.  See
the "Additional build tips" section of the Making LAMMPS documentation
pages for workarounds on this issue.

[Related commands:]

"pair_coeff"_pair_coeff.html

[Default:] none

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:link(MacKerell)
[(MacKerell)] MacKerell, Bashford, Bellott, Dunbrack, Evanseck, Field,
Fischer, Gao, Guo, Ha, et al, J Phys Chem, 102, 3586 (1998).