"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 fix qeq/point command :h3 fix qeq/shielded command :h3 fix qeq/slater command :h3 fix qeq/dynamic command :h3 [Syntax:] fix ID group-ID style Nevery cutoff tolerance maxiter qfile :pre ID, group-ID are documented in "fix"_fix.html command style = {qeq/point} or {qeq/shielded} or {qeq/slater} or {qeq/dynamic} Nevery = perform charge equilibration every this many steps cutoff = global cutoff for charge-charge interactions (distance unit) tolerance = precision to which charges will be equilibrated maxiter = maximum iterations to perform charge equilibration qfile = a filename with QEq parameters :ul [Examples:] fix 1 all qeq/point 1 10 1.0e-6 200 param.qeq1 fix 1 qeq qeq/shielded 1 8 1.0e-6 100 param.qeq2 fix 1 all qeq/slater 5 10 1.0e-6 100 params fix 1 qeq qeq/dynamic 1 12 1.0e-3 100 my_qeq :pre [Description:] Perform the charge equilibration (QEq) method as described in "(Rappe and Goddard)"_#Rappe and formulated in "(Nakano)"_#Nakano (also known as the matrix inversion method) and in "(Rick and Stuart)"_#Rick (also known as the extended Lagrangian method) based on the electronegativity equilization principle. These fixes can be used with any "pair style"_pair_style.html in LAMMPS, so long as per-atom charges are defined. The most typical use-case is in conjunction with a "pair style"_pair_style.html that performs charge equilibration periodically (e.g. every timestep), such as the ReaxFF or Streitz-Mintmire potential (the latter is not yet implemented in LAMMPS). But these fixes can also be used with potentials that normally assume per-atom charges are fixed, e.g. a "Buckingham"_pair_buck.html or "LJ/Coulombic"_pair_lj.html potential. Because the charge equilibration calculation is effectively independent of the pair style, these fixes can also be used to perform a one-time assignment of charges to atoms. For example, you could define the QEq fix, perform a zero-timestep run via the "run"_run.html command without any pair style defined which would set per-atom charges (based on the current atom configuration), then remove the fix via the "unfix"_unfix.html command before performing further dynamics. IMPORTANT NOTE: Computing and using charge values different from published values defined for a fixed-charge potential like Buckingham or CHARMM or AMBER, can have a strong effect on energies and forces, and produces a different model than the published versions. IMPORTANT NOTE: The "fix qeq/comb"_fix_qeq_comb.html command must still be used to perform charge equliibration with the "COMB potential"_pair_comb.html. The "fix qeq/reax"_fix_qeq_reax.html command can be used to perform charge equilibration with the "ReaxFF force field"_pair_reax_c.html, although fix qeq/shielded yields the same results as fix qeq/reax if {Nevery}, {cutoff}, and {tolerance} are the same. Eventually the fix qeq/reax command will be deprecated. The QEq method minimizes the electrostatic energy of the system (or equalizes the derivative of energy with respect to charge of all the atoms) by adjusting the partial charge on individual atoms based on interactions with their neighbors within {cutoff}. It reqires a few parameters, in {metal} units, for each atom type which provided in a file specified by {qfile}. The file has the following format 1 chi eta gamma zeta qcore 2 chi eta gamma zeta qcore ... Ntype chi eta gamma zeta qcore :pre There is one line per atom type with the following parameters. Only a subset of the parameters is used by each QEq style as descibed below, thus the others can be set to 0.0 if desired. {chi} = electronegativity in energy units {eta} = self-Coulomb potential in energy units {gamma} = shielded Coulomb constant defined by "ReaxFF force field"_#vanDuin in distance units {zeta} = Slater type orbital exponent defined by the "Streitz-Mintmire"_#Streitz potential in reverse distance units {qcore} = charge of the nucleus defined by the "Streitz-Mintmire potential"_#Streitz potential in charge units :ul The {qeq/point} style describes partial charges on atoms as point charges. Interaction between a pair of charged particles is 1/r, which is the simplest description of the interaction between charges. Only the {chi} and {eta} parameters from the {qfile} file are used. Note that Coulomb catastrophe can occur if repulsion between the pair of charged particles is too weak. This style solves partial charges on atoms via the matrix inversion method. A tolerance of 1.0e-6 is usually a good number. The {qeq/shielded} style describes partial charges on atoms also as point charges, but uses a shielded Coulomb potential to describe the interaction between a pair of charged particles. Interaction through the shielded Coulomb is given by equation (13) of the "ReaxFF force field"_#vanDuin paper. The shielding accounts for charge overlap between charged particles at small separation. This style is the same as "fix qeq/reax"_fix_qeq_reax.html, and can be used with "pair_style reax/c"_pair_reax_c.html. Only the {chi}, {eta}, and {gamma} parameters from the {qfile} file are used. This style solves partial charges on atoms via the matrix inversion method. A tolerance of 1.0e-6 is usually a good number. The {qeq/slater} style describes partial charges on atoms as spherical charge densities centered around atoms via the Slater 1{s} orbital, so that the interaction between a pair of charged particles is the product of two Slater 1{s} orbitals. The expression for the Slater 1{s} orbital is given under equation (6) of the "Streitz-Mintmire"_#Streitz paper. Only the {chi}, {eta}, {zeta}, and {qcore} parameters from the {qfile} file are used. This style solves partial charges on atoms via the matrix inversion method. A tolerance of 1.0e-6 is usually a good number. The {qeq/dynamic} style describes partial charges on atoms as point charges that interact through 1/r, but the extended Lagrangian method is used to solve partial charges on atoms. Only the {chi} and {eta} parameters from the {qfile} file are used. Note that Coulomb catastrophe can occur if repulsion between the pair of charged particles is too weak. A tolerance of 1.0e-3 is usually a good number. Note that {qeq/point}, {qeq/shielded}, and {qeq/slater} describe different charge models, whereas the matrix inversion method and the extended Lagrangian method ({qeq/dynamic}) are different solvers. Note that the {qeq/point} and the {qeq/dynamic} styles both describe charges as point charges that interact through 1/r relationship, but solve partial charges on atoms using different solvers. Styles {qeq/point} and the {qeq/dynamic} should yield comparable results if the QEq parameters and {Nevery}, {cutoff}, and {tolerance} are the same. Style {qeq/point} is typically faster, but {qeq/dynamic} scales better on larger sizes. IMPORTANT NOTE: To avoid the evaluation of the derivative of charge with respect to position, which is typically ill-defined, the system should have a zero net charge. IMPORTANT NOTE: Developing QEq parameters (chi, eta, gamma, zeta, and qcore) is an "art". Charges on atoms are not guaranteed to equilibrate with arbitrary choices of these parameters. We do not develop these QEq paramters. See the examples/qeq directory for some examples. [Restart, fix_modify, output, run start/stop, minimize info:] No information about these fixes is written to "binary restart files"_restart.html. No global scalar or vector or per-atom quantities are stored by these fixes for access by various "output commands"_Section_howto.html#howto_15. No parameter of these fixes can be used with the {start/stop} keywords of the "run"_run.html command. Thexe fixes are invoked during "energy minimization"_minimize.html. [Restrictions:] These fixes are part of the QEQ 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:] "fix qeq/reax"_fix_qeq_reax.html, "fix qeq/comb"_fix_qeq_comb.html [Default:] none :line :link(Rappe) [(Rappe and Goddard)] A. K. Rappe and W. A. Goddard III, J Physical Chemistry, 95, 3358-3363 (1991). :link(Nakano) [(Nakano)] A. Nakano, Computer Physics Communications, 104, 59-69 (1997). :link(Rick) [(Rick and Stuart)] S. W. Rick, S. J. Stuart, B. J. Berne, J Chemical Physics 101, 16141 (1994). :link(Streitz) [(Streitz-Mintmire)] F. H. Streitz, J. W. Mintmire, Physical Review B, 50, 16, 11996 (1994) :link(vanDuin) [(ReaxFF)] A. C. T. van Duin, S. Dasgupta, F. Lorant, W. A. Goddard III, J Physical Chemistry, 105, 9396-9049 (2001)