.. index:: pair_style lj/cut/dipole/cut pair_style lj/cut/dipole/cut command ==================================== pair_style lj/cut/dipole/cut/gpu command ======================================== pair_style lj/cut/dipole/cut/omp command ======================================== pair_style lj/sf/dipole/sf command ================================== pair_style lj/sf/dipole/sf/gpu command ====================================== pair_style lj/sf/dipole/sf/omp command ====================================== pair_style lj/cut/dipole/long command ===================================== pair_style lj/long/dipole/long command ====================================== Syntax """""" .. parsed-literal:: pair_style lj/cut/dipole/cut cutoff (cutoff2) pair_style lj/sf/dipole/sf cutoff (cutoff2) pair_style lj/cut/dipole/long cutoff (cutoff2) pair_style lj/long/dipole/long flag_lj flag_coul cutoff (cutoff2) * cutoff = global cutoff LJ (and Coulombic if only 1 arg) (distance units) * cutoff2 = global cutoff for Coulombic and dipole (optional) (distance units) * flag_lj = *long* or *cut* or *off* .. parsed-literal:: *long* = use long-range damping on dispersion 1/r^6 term *cut* = use a cutoff on dispersion 1/r^6 term *off* = omit disperion 1/r^6 term entirely * flag_coul = *long* or *off* .. parsed-literal:: *long* = use long-range damping on Coulombic 1/r and point-dipole terms *off* = omit Coulombic and point-dipole terms entirely Examples """""""" .. parsed-literal:: pair_style lj/cut/dipole/cut 10.0 pair_coeff * * 1.0 1.0 pair_coeff 2 3 1.0 1.0 2.5 4.0 .. parsed-literal:: pair_style lj/sf/dipole/sf 9.0 pair_coeff * * 1.0 1.0 pair_coeff 2 3 1.0 1.0 2.5 4.0 .. parsed-literal:: pair_style lj/cut/dipole/long 10.0 pair_coeff * * 1.0 1.0 pair_coeff 2 3 1.0 1.0 2.5 4.0 .. parsed-literal:: pair_style lj/long/dipole/long long long 3.5 10.0 pair_coeff * * 1.0 1.0 pair_coeff 2 3 1.0 1.0 2.5 4.0 Description """"""""""" Style *lj/cut/dipole/cut* computes interactions between pairs of particles that each have a charge and/or a point dipole moment. In addition to the usual Lennard-Jones interaction between the particles (Elj) the charge-charge (Eqq), charge-dipole (Eqp), and dipole-dipole (Epp) interactions are computed by these formulas for the energy (E), force (F), and torque (T) between particles I and J. .. image:: Eqs/pair_dipole.jpg :align: center where qi and qj are the charges on the two particles, pi and pj are the dipole moment vectors of the two particles, r is their separation distance, and the vector r = Ri - Rj is the separation vector between the two particles. Note that Eqq and Fqq are simply Coulombic energy and force, Fij = -Fji as symmetric forces, and Tij != -Tji since the torques do not act symmetrically. These formulas are discussed in :ref:`(Allen) ` and in :ref:`(Toukmaji) `. Style *lj/sf/dipole/sf* computes "shifted-force" interactions between pairs of particles that each have a charge and/or a point dipole moment. In general, a shifted-force potential is a (sligthly) modified potential containing extra terms that make both the energy and its derivative go to zero at the cutoff distance; this removes (cutoff-related) problems in energy conservation and any numerical instability in the equations of motion :ref:`(Allen) `. Shifted-force interactions for the Lennard-Jones (E_LJ), charge-charge (Eqq), charge-dipole (Eqp), dipole-charge (Epq) and dipole-dipole (Epp) potentials are computed by these formulas for the energy (E), force (F), and torque (T) between particles I and J: .. image:: Eqs/pair_dipole_sf.jpg :align: center .. image:: Eqs/pair_dipole_sf2.jpg :align: center where epsilon and sigma are the standard LJ parameters, r_c is the cutoff, qi and qj are the charges on the two particles, pi and pj are the dipole moment vectors of the two particles, r is their separation distance, and the vector r = Ri - Rj is the separation vector between the two particles. Note that Eqq and Fqq are simply Coulombic energy and force, Fij = -Fji as symmetric forces, and Tij != -Tji since the torques do not act symmetrically. The shifted-force formula for the Lennard-Jones potential is reported in :ref:`(Stoddard) `. The original (unshifted) formulas for the electrostatic potentials, forces and torques can be found in :ref:`(Price) `. The shifted-force electrostatic potentials have been obtained by applying equation 5.13 of :ref:`(Allen) `. The formulas for the corresponding forces and torques have been obtained by applying the 'chain rule' as in appendix C.3 of :ref:`(Allen) `. If one cutoff is specified in the pair_style command, it is used for both the LJ and Coulombic (q,p) terms. If two cutoffs are specified, they are used as cutoffs for the LJ and Coulombic (q,p) terms respectively. Style *lj/cut/dipole/long* computes long-range point-dipole interactions as discussed in :ref:`(Toukmaji) `. Dipole-dipole, dipole-charge, and charge-charge interactions are all supported, along with the standard 12/6 Lennard-Jones interactions, which are computed with a cutoff. A :doc:`kspace_style ` must be defined to use this pair style. Currently, only :doc:`kspace_style ewald/disp ` support long-range point-dipole interactions. Style *lj/long/dipole/long* also computes point-dipole interactions as discussed in :ref:`(Toukmaji) `. Long-range dipole-dipole, dipole-charge, and charge-charge interactions are all supported, along with the standard 12/6 Lennard-Jones interactions. LJ interactions can be cutoff or long-ranged. For style *lj/long/dipole/long*\ , if *flag_lj* is set to *long*\ , no cutoff is used on the LJ 1/r^6 dispersion term. The long-range portion is calculated by using the :doc:`kspace_style ewald_disp ` command. The specified LJ cutoff then determines which portion of the LJ interactions are computed directly by the pair potential versus which part is computed in reciprocal space via the Kspace style. If *flag_lj* is set to *cut*\ , the LJ interactions are simply cutoff, as with :doc:`pair_style lj/cut `. If *flag_lj* is set to *off*\ , LJ interactions are not computed at all. If *flag_coul* is set to *long*\ , no cutoff is used on the Coulombic or dipole interactions. The long-range portion is calculated by using *ewald_disp* of the :doc:`kspace_style ` command. If *flag_coul* is set to *off*\ , Coulombic and dipole interactions are not computed at all. Atoms with dipole moments should be integrated using the :doc:`fix nve/sphere update dipole ` command to rotate the dipole moments. The *omega* option on the :doc:`fix langevin ` command can be used to thermostat the rotational motion. The :doc:`compute temp/sphere ` command can be used to monitor the temperature, since it includes rotational degrees of freedom. The :doc:`atom_style dipole ` command should be used since it defines the point dipoles and their rotational state. The magnitude of the dipole moment for each type of particle can be defined by the :doc:`dipole ` command or in the "Dipoles" section of the data file read in by the :doc:`read_data ` command. Their initial orientation can be defined by the :doc:`set dipole ` command or in the "Atoms" section of the data file. The following coefficients must be defined for each pair of atoms types via the :doc:`pair_coeff ` command as in the examples above, or in the data file or restart files read by the :doc:`read_data ` or :doc:`read_restart ` commands, or by mixing as described below: * epsilon (energy units) * sigma (distance units) * cutoff1 (distance units) * cutoff2 (distance units) 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. ---------- Styles with a *cuda*\ , *gpu*\ , *intel*\ , *kk*\ , *omp*\ , 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 :doc:`Section_accelerate ` 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, USER-INTEL, KOKKOS, USER-OMP and OPT packages, respectively. They are only enabled if LAMMPS was built with those packages. See the :ref:`Making LAMMPS ` section for more info. You can specify the accelerated styles explicitly in your input script by including their suffix, or you can use the :ref:`-suffix command-line switch ` when you invoke LAMMPS, or you can use the :doc:`suffix ` command in your input script. See :doc:`Section_accelerate ` of the manual for more instructions on how to use the accelerated styles effectively. ---------- **Mixing, shift, table, tail correction, restart, rRESPA info**\ : For atom type pairs I,J and I != J, the epsilon and sigma coefficients and cutoff distances for this pair style can be mixed. The default mix value is *geometric*\ . See the "pair_modify" command for details. For atom type pairs I,J and I != J, the A, sigma, d1, and d2 coefficients and cutoff distance for this pair style can be mixed. A is an energy value mixed like a LJ epsilon. D1 and d2 are distance values and are mixed like sigma. The default mix value is *geometric*\ . See the "pair_modify" command for details. This pair style does not support the :doc:`pair_modify ` shift option for the energy of the Lennard-Jones portion of the pair interaction; such energy goes to zero at the cutoff by construction. The :doc:`pair_modify ` table option is not relevant for this pair style. This pair style does not support the :doc:`pair_modify ` tail option for adding long-range tail corrections to energy and pressure. This pair style writes its information to :doc:`binary restart files `, so pair_style and pair_coeff commands do not need to be specified in an input script that reads a restart file. This pair style can only be used via the *pair* keyword of the :doc:`run_style respa ` command. It does not support the *inner*\ , *middle*\ , *outer* keywords. Restrictions """""""""""" The *lj/cut/dipole/cut*\ , *lj/cut/dipole/long*\ , and *lj/long/dipole/long* styles are part of the DIPOLE package. They are only enabled if LAMMPS was built with that package. See the :ref:`Making LAMMPS ` section for more info. The *lj/sf/dipole/sf* style is part of the USER-MISC package. It is only enabled if LAMMPS was built with that package. See the :ref:`Making LAMMPS ` section for more info. Using dipole pair styles with *electron* :doc:`units ` is not currently supported. Related commands """""""""""""""" :doc:`pair_coeff ` **Default:** none ---------- .. _Allen: **(Allen)** Allen and Tildesley, Computer Simulation of Liquids, Clarendon Press, Oxford, 1987. .. _Toukmaji: **(Toukmaji)** Toukmaji, Sagui, Board, and Darden, J Chem Phys, 113, 10913 (2000). .. _Stoddard: **(Stoddard)** Stoddard and Ford, Phys Rev A, 8, 1504 (1973). .. _Price: **(Price)** Price, Stone and Alderton, Mol Phys, 52, 987 (1984). .. _lws: http://lammps.sandia.gov .. _ld: Manual.html .. _lc: Section_commands.html#comm