"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 buck6d/coul/gauss/dsf :h3
pair_style buck6d/coul/gauss/long :h3

[Syntax:]

pair_style style args :pre

style = {buck6d/coul/gauss/dsf} or {buck6d/coul/gauss/long}
args = list of arguments for a particular style :ul
  {buck6d/coul/gauss/dsf} args = smooth cutoff (cutoff2)
    smooth  = smoothing onset within Buckingham cutoff (ratio)
    cutoff  = global cutoff for Buckingham (and Coulombic if only 1 arg) (distance units)
    cutoff2 = global cutoff for Coulombic (optional) (distance units)
  {buck6d/coul/gauss/long} args = smooth smooth2 cutoff (cutoff2)
    smooth   = smoothing onset within Buckingham cutoff (ratio)
    smooth2  = smoothing onset within Coulombic cutoff (ratio)
    cutoff   = global cutoff for Buckingham (and Coulombic if only 1 arg) (distance units)
    cutoff2  = global cutoff for Coulombic (optional) (distance units) :pre

[Examples:]

pair_style buck6d/coul/gauss/dsf    0.9000    12.0000
pair_coeff 1  1  1030.  3.061  457.179  4.521  0.608 :pre

pair_style buck6d/coul/gauss/long   0.9000  1.0000  12.0000
pair_coeff 1  1  1030.  3.061  457.179  4.521  0.608 :pre

[Description:]

The {buck6d/coul/gauss} styles evaluate vdW and Coulomb
interactions following the MOF-FF force field after
"(Schmid)"_#Schmid. The vdW term of the {buck6d} styles 
computes a dispersion damped Buckingham potential:

:c,image(Eqs/pair_buck6d.jpg)

where A and C are a force constant, kappa is an ionic-pair dependent 
reciprocal length parameter, D is a dispersion correction parameter, 
and the cutoff Rc truncates the interaction distance.
The first term in the potential corresponds to the Buckingham 
repulsion term and the second term to the dispersion attraction with 
a damping correction analog to the Grimme correction used in DFT.  
The latter corrects for artifacts occurring at short distances which 
become an issue for soft vdW potentials. 

The {buck6d} styles include a smoothing function which is invoked 
according to the global smooting parameter within the specified 
cutoff.  Hereby a parameter of i.e. 0.9 invokes the smoothing 
within 90% of the cutoff.  No smoothing is applied at a value
of 1.0. For the {gauss/dsf} style this smoothing is only applicable 
for the dispersion damped Buckingham potential. For the {gauss/long}
styles the smoothing function can also be invoked for the real
space coulomb interactions which enforce continous energies and 
forces at the cutoff.

Both styles {buck6d/coul/gauss/dsf} and {buck6d/coul/gauss/long} 
evaluate a Coulomb potential using spherical Gaussian type charge 
distributions which effectively dampen electrostatic ineractions 
for high charges at close distances.  The electrostatic potential
is thus evaluated as:

:c,image(Eqs/pair_coul_gauss.jpg)

where C is an energy-conversion constant, Qi and Qj are the 
charges on the 2 atoms, epsilon is the dielectric constant which 
can be set by the "dielectric"_dielectric.html command, alpha is 
ion pair dependent damping parameter and erf() is the error-function.  
The cutoff Rc truncates the interaction distance.

The style {buck6d/coul/gauss/dsf} computes the Coulomb interaction
via the damped shifted force model described in "(Fennell)"_#Fennell 
approximating an Ewald sum similar to the "pair coul/dsf"_pair_coul.html
styles. In {buck6d/coul/gauss/long} an additional damping factor is 
applied to the Coulombic term so it can be used in conjunction with the 
"kspace_style"_kspace_style.html command and its {ewald} or {pppm} 
options. The Coulombic cutoff in this case separates the real and 
reciprocal space evaluation of the Ewald sum.

If one cutoff is specified it is used for both the vdW and Coulomb
terms.  If two cutoffs are specified, the first is used as the cutoff 
for the vdW terms, and the second is the cutoff for the Coulombic term.

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:

A (energy units)
rho (distance^-1 units)
C (energy-distance^6 units)
D (distance^14 units)
alpha (distance^-1 units)
cutoff (distance units) :ul

The second coefficient, rho, must be greater than zero. The latter 
coefficient is optional.  If not specified, the global vdW cutoff 
is used. 

:line

[Mixing, shift, table, tail correction, restart, rRESPA info]:

These pair styles do not support mixing.  Thus, coefficients for all
I,J pairs must be specified explicitly.

These styles do not support the "pair_modify"_pair_modify.html shift
option for the energy. Instead the smoothing function should be applied
by setting the global smoothing parameter to a value < 1.0.

These 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.

[Restrictions:]

These styles are part of the USER-MOFFF package.  They are only enabled 
if LAMMPS was built with that package.  See the
"Making LAMMPS"_Section_start.html#start_3 section for more info.

[Related commands:]

"pair_coeff"_pair_coeff.html

[Default:] none

:link(Schmid)
[(Schmid)] S. Bureekaew, S. Amirjalayer, M. Tafipolsky, C. Spickermann, T.K. Roy and R. Schmid, Phys. Status Solidi B, 6, 1128 (2013).
:link(Fennell)
[(Fennell)] C. J. Fennell, J. D. Gezelter, J Chem Phys, 124, 234104 (2006).
