forked from lijiext/lammps
205 lines
8.2 KiB
Plaintext
Executable File
205 lines
8.2 KiB
Plaintext
Executable File
"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 dipole/cut command :h3
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pair_style dipole/cut/gpu command :h3
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pair_style dipole/cut/omp command :h3
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pair_style dipole/sf command :h3
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pair_style dipole/sf/gpu command :h3
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pair_style dipole/sf/omp command :h3
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[Syntax:]
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pair_style dipole/cut cutoff (cutoff2) :pre
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pair_style dipole/sf cutoff (cutoff2) :pre
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cutoff = global cutoff LJ (and Coulombic if only 1 arg) (distance units)
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cutoff2 = global cutoff for Coulombic (optional) (distance units) :ul
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[Examples:]
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pair_style dipole/cut 10.0
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pair_coeff * * 1.0 1.0
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pair_coeff 2 3 1.0 1.0 2.5 4.0 :pre
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pair_style dipole/sf 9.0
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pair_coeff * * 1.0 1.0
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pair_coeff 2 3 1.0 1.0 2.5 4.0 :pre
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[Description:]
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Style {dipole/cut} computes interactions between pairs of particles
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that each have a charge and/or a point dipole moment. In addition to
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the usual Lennard-Jones interaction between the particles (Elj) the
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charge-charge (Eqq), charge-dipole (Eqp), and dipole-dipole (Epp)
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interactions are computed by these formulas for the energy (E), force
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(F), and torque (T) between particles I and J.
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:c,image(Eqs/pair_dipole.jpg)
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where qi and qj are the charges on the two particles, pi and pj are
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the dipole moment vectors of the two particles, r is their separation
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distance, and the vector r = Ri - Rj is the separation vector between
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the two particles. Note that Eqq and Fqq are simply Coulombic energy
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and force, Fij = -Fji as symmetric forces, and Tij != -Tji since the
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torques do not act symmetrically. These formulas are discussed in
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"(Allen)"_#Allen and in "(Toukmaji)"_#Toukmaji.
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Style {dipole/sf} computes "shifted-force" interactions between pairs
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of particles that each have a charge and/or a point dipole moment. In
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general, a shifted-force potential is a (sligthly) modified potential
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containing extra terms that make both the energy and its derivative go
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to zero at the cutoff distance; this removes (cutoff-related) problems
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in energy conservation and any numerical instability in the equations
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of motion "(Allen)"_#Allen. Shifted-force interactions for the
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Lennard-Jones (E_LJ), charge-charge (Eqq), charge-dipole (Eqp),
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dipole-charge (Epq) and dipole-dipole (Epp) potentials are computed by
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these formulas for the energy (E), force (F), and torque (T) between
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particles I and J:
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:c,image(Eqs/pair_dipole_sf.jpg)
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:c,image(Eqs/pair_dipole_sf2.jpg)
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where epsilon and sigma are the standard LJ parameters, r_c is the
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cutoff, qi and qj are the charges on the two particles, pi and pj are
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the dipole moment vectors of the two particles, r is their separation
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distance, and the vector r = Ri - Rj is the separation vector between
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the two particles. Note that Eqq and Fqq are simply Coulombic energy
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and force, Fij = -Fji as symmetric forces, and Tij != -Tji since the
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torques do not act symmetrically. The shifted-force formula for the
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Lennard-Jones potential is reported in "(Stoddard)"_#Stoddard. The
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original (unshifted) formulas for the electrostatic potentials, forces
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and torques can be found in "(Price)"_#Price. The shifted-force
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electrostatic potentials have been obtained by applying equation 5.13
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of "(Allen)"_#Allen. The formulas for the corresponding forces and
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torques have been obtained by applying the 'chain rule' as in appendix
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C.3 of "(Allen)"_#Allen.
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If one cutoff is specified in the pair_style command, it is used for
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both the LJ and Coulombic (q,p) terms. If two cutoffs are specified,
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they are used as cutoffs for the LJ and Coulombic (q,p) terms
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respectively.
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Atoms with dipole moments should be integrated using the "fix
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nve/sphere update dipole"_fix_nve_sphere.html command to rotate the
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dipole moments. The "compute temp/sphere"_compute_temp_sphere.html
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command can be used to monitor the temperature, since it includes
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rotational degrees of freedom. The "atom_style
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dipole"_atom_style.html command should be used since it defines the
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point dipoles and their rotational state. The magnitude of the dipole
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moment for each type of particle can be defined by the
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"dipole"_dipole.html command or in the "Dipoles" section of the data
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file read in by the "read_data"_read_data.html command. Their initial
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orientation can be defined by the "set dipole"_set.html command or in
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the "Atoms" section of the data file.
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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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cutoff1 (distance units)
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cutoff2 (distance units) :ul
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The latter 2 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.
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:line
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Styles with a {cuda}, {gpu}, {omp}, or {opt} suffix are functionally
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the same as the corresponding style without the suffix. They have
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been optimized to run faster, depending on your available hardware, as
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discussed in "Section_accelerate"_Section_accelerate.html of the
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manual. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
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packages, respectively. They are only enabled if LAMMPS was built with
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those packages. See the "Making LAMMPS"_Section_start.html#start_3
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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, table, tail correction, restart, rRESPA 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 distances for this pair style can be mixed. The default
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mix value is {geometric}. See the "pair_modify" command for details.
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For atom type pairs I,J and I != J, the A, sigma, d1, and d2
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coefficients and cutoff distance for this pair style can be mixed. A
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is an energy value mixed like a LJ epsilon. D1 and d2 are distance
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values and are mixed like sigma. The default mix value is
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{geometric}. See the "pair_modify" command for details.
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This pair style does not support the "pair_modify"_pair_modify.html
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shift option for the energy of the Lennard-Jones portion of the pair
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interaction; such energy goes to zero at the cutoff by construction.
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The "pair_modify"_pair_modify.html table option is not relevant
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for this pair style.
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This pair style does not support the "pair_modify"_pair_modify.html
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tail option for adding long-range tail corrections to energy and
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pressure.
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This pair style writes its 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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This pair style can only be used via the {pair} keyword of the
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"run_style respa"_run_style.html command. It does not support the
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{inner}, {middle}, {outer} keywords.
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[Restrictions:]
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The {dipole/cut} style is part of the DIPOLE package. It is only
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enabled if LAMMPS was built with that package. See the "Making
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LAMMPS"_Section_start.html#start_3 section for more info.
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The {dipole/sf} style is part of the USER-MISC package. It is only
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enabled if LAMMPS was built with that package. See the "Making
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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
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[Default:] none
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:line
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:link(Allen)
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[(Allen)] Allen and Tildesley, Computer Simulation of Liquids,
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Clarendon Press, Oxford, 1987.
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:link(Toukmaji)
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[(Toukmaji)] Toukmaji, Sagui, Board, and Darden, J Chem Phys, 113,
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10913 (2000).
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:link(Stoddard)
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[(Stoddard)] Stoddard and Ford, Phys Rev A, 8, 1504 (1973).
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:link(Price)
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[(Price)] Price, Stone and Alderton, Mol Phys, 52, 987 (1984).
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