Qeq Reax
Giorgina Makara
Perform the charge equilibration (QEq) method as described in (Rappeand Goddard, 1991) and formulated in (Nakano,1997). It is typically used in conjunction with theReaxFF force field model as implemented in the pair_stylereax/c command, but it can be used with anypotential in LAMMPS, so long as it defines and uses charges on eachatom. The fix qeq/comb command should be used toperform charge equliibration with the COMB potential.For more technical details about the charge equilibration performed byfix qeq/reax, see the "(Aktulga)" paper.
The QEq method minimizes the electrostatic energy of the system byadjusting the partial charge on individual atoms based on interactionswith their neighbors. It reqires some parameters for each atom type.If the params setting above is the word "reax/c", then these areextracted from the pair_style reax/c command andthe ReaxFF force field file it reads in. If a file name is specifiedfor params, then the parameters are taken from the specified fileand the file must contain one line for each atom type. The latterform must be used when performing QeQ with a non-ReaxFF potential.Each line should be formatted as follows:
where itype is the atom type from 1 to Ntypes, chi denotes theelectronegativity in eV, eta denotes the self-Coulombpotential in eV, and gamma denotes the valence orbitalexponent. Note that these 3 quantities are also in the ReaxFFpotential file, except that eta is defined here as twice the eta valuein the ReaxFF file. Note that unlike the rest of LAMMPS, the unitsof this fix are hard-coded to be A, eV, and electronic charge.
No information about this fix is written to binary restartfiles. No global scalar or vector or per-atomquantities are stored by this fix for access by various outputcommands. No parameter of this fix canbe used with the start/stop keywords of the run command.
Perform the charge equilibration (QEq) method as described in(Rappe and Goddard) and formulated in (Nakano). It is typically used in conjunction with the ReaxFF forcefield model as implemented in the pair_style reaxffcommand, but it can be used with any potential in LAMMPS, so long as itdefines and uses charges on each atom. The fix qeq/comb command should be used to perform charge equilibrationwith the COMB potential. For more technical detailsabout the charge equilibration performed by fix qeq/reaxff, see the(Aktulga) paper.
where itype is the atom type from 1 to Ntypes, chi denotes theelectronegativity in eV, eta denotes the self-Coulombpotential in eV, and gamma denotes the valence orbitalexponent. Note that these 3 quantities are also in the ReaxFFpotential file, except that eta is defined here as twice the eta valuein the ReaxFF file. Note that unlike the rest of LAMMPS, the unitsof this fix are hard-coded to be A, eV, and electronic charge.
The optional dual keyword allows to perform the optimizationof the S and T matrices in parallel. This is only supported forthe qeq/reaxff/omp style. Otherwise they are processed separately.The qeq/reaxff/kk style always solves the S and T matrices inparallel.
The optional nowarn keyword silences the warning message printedwhen the maximum number of iterations was reached. This can beuseful for comparing serial and parallel results where having thesame fixed number of QEq iterations is desired, which can be achievedby using a very small tolerance and setting maxiter to the desirednumber of iterations.
In order to solve the self-consistent equations for electronegativityequalization, LAMMPS imposes the additional constraint that all thecharges in the fix group must add up to zero. The initial chargeassignments should also satisfy this constraint. LAMMPS will print awarning if that is not the case.
No information about this fix is written to binary restart files. This fix computes a global scalar (the number ofiterations) for access by various output commands.No parameter of this fix can be used with the start/stop keywords ofthe run command.
Styles with a gpu, intel, kk, omp, or opt suffix arefunctionally the same as the corresponding style without the suffix.They have been optimized to run faster, depending on your availablehardware, as discussed on the Accelerator packagespage. The accelerated styles take the same arguments and shouldproduce the same results, except for round-off and precision issues.
These accelerated styles are part of the GPU, INTEL, KOKKOS,OPENMP, and OPT packages, respectively. They are only enabled ifLAMMPS was built with those packages. See the Build package page for more info.
You can specify the accelerated styles explicitly in your input scriptby including their suffix, or you can use the -suffix command-line switch when you invoke LAMMPS, or you can use thesuffix command in your input script.
This fix may be used in combination with fix efieldand will apply the external electric field during charge equilibration,but there may be only one fix efield instance used and the electric fieldvector may only have components in non-periodic directions. Equal-stylevariables can be used for electric field vector components without any furthersettings. Atom-style variables can be used for spatially-varying electric fieldvector components, but the resulting electric potential must be specifiedas an atom-style variable using the potential keyword for fix efield.
Perform the charge equilibration (QEq) method as described in (Rappe and Goddard) and formulated in (Nakano). It istypically used in conjunction with the ReaxFF force field model asimplemented in the pair_style reax/c command, butit can be used with any potential in LAMMPS, so long as it defines anduses charges on each atom. The fix qeq/combcommand should be used to perform charge equilibration with the COMB potential. For more technical details about thecharge equilibration performed by fix qeq/reax, see the(Aktulga) paper.
No information about this fix is written to binary restart files. No global scalar or vector or per-atomquantities are stored by this fix for access by various output commands. No parameter of this fix can be usedwith the start/stop keywords of the run command.
Styles with a gpu, intel, kk, omp, or opt suffix arefunctionally the same as the corresponding style without the suffix.They have been optimized to run faster, depending on your availablehardware, as discussed on the Speed packages docpage. The accelerated styles take the same arguments and shouldproduce the same results, except for round-off and precision issues.
These accelerated styles are part of the GPU, USER-INTEL, KOKKOS,USER-OMP and OPT packages, respectively. They are only enabled ifLAMMPS was built with those packages. See the Build package doc page for more info.
It would be so appreciated if you could explain how obtain the parameters in param file in fix qeq/reax. One can get the second and third columns of this file from DFT calculations using chi=1/2(IP+EA) and eta=IP-EA equations, but what about valence orbital exponent gamma?
Thank you very much in advance,
If you want to use the fix QEQ1 all qeq/reaxff 1 0.0 10.0 1.0e-6 param.qeq command, you will need to provide a file called param.qeq and carefully specify the values for itype chi eta gamma . These values can be found in the force-field file :
I fixed my temperature at 330K with an equilibration run of 120ps during which I get the WARNING: Fix qeq/reax CG convergence failed after 200 iterations at 1 step and the temperature, kinetic energy and pressure outputs nan on screen as shown in the log.lammps file.
I used packmol to solvate the ethylene carbonate and lithium hexafluorophosphate which generated a .pdb file I viewed with VNL. I tried to save the solvate as a lammps data but VNL could only save it as atomic atom_style data with the charge (q) column missing, while I am using charge atom_style for my simulation.
Perform the charge equilibration (QEq) method as described in(Rappe and Goddard) and formulated in (Nakano) (also known as the matrix inversion method) and in(Rick and Stuart) (also known as the extended Lagrangianmethod) based on the electronegativity equilization principle.
These fixes can be used with any pair style inLAMMPS, so long as per-atom charges are defined. The most typicaluse-case is in conjunction with a pair style thatperforms charge equilibration periodically (e.g. every timestep), suchas the ReaxFF or Streitz-Mintmire potential. But these fixes can alsobe used with potentials that normally assume per-atom charges are fixed,e.g. a Buckingham or LJ/Coulombicpotential.
Because the charge equilibration calculation is effectively independentof the pair style, these fixes can also be used to perform a one-timeassignment of charges to atoms. For example, you could define the QEqfix, perform a zero-timestep run via the run commandwithout any pair style defined which would set per-atom charges (basedon the current atom configuration), then remove the fix via theunfix command before performing further dynamics.
Computing and using charge values different from publishedvalues defined for a fixed-charge potential like Buckingham or CHARMMor AMBER, can have a strong effect on energies and forces, andproduces a different model than the published versions.
The fix qeq/comb command must still be used toperform charge equilibration with the COMB potential. The fix qeq/reaxff command can beused to perform charge equilibration with the ReaxFF forcefield, although fix qeq/shielded yields the sameresults as fix qeq/reaxff if Nevery, cutoff, and toleranceare the same. Eventually the fix qeq/reaxff command will bedeprecated.
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