Electrostatic potentials¶

exaStamp computes Coulomb interactions either with a cutoff (short range methods) or with the full periodic sum (long range methods).

Method	Operators	Cost	Use it for
Plain cutoff	`coul_cut` (pair style)	\(O(N)\)	Screened or dilute systems only
Damped shifted force	`coulombic_dsf`, `coul_dsf`	\(O(N)\)	Ionic systems, good energy conservation
Wolf summation	`coulombic_wolf`, `coul_wolf`	\(O(N)\)	Ionic systems
Reaction field	`coulombic_rf`, `coul_rf`	\(O(N)\)	Polar molecules in a dielectric medium
Ewald summation	`coulombic_ewald_init`, `coulombic_ewald_short_range`, `coulombic_ewald_long_range`	\(O(N \cdot N_k)\)	Exact periodic sum, small systems
PPPM	`coulombic_pppm_init`, `coulombic_ewald_short_range`, `coulombic_pppm`	\(O(N + M\log M)\)	Exact periodic sum, large systems

Two families of operators exist:

coulombic_* operators read per-particle charges (the charge field) by default, or the species charges with per_atom_charge: false.
Pair styles (coul_cut, coul_dsf, coul_wolf, coul_rf, and the combined ljwolf, ljrf, exp6rf, ljexp6rf) use the species charges and support the multi-species strategies of the pair potential template.

Charge related helper operators:

Operator	Role
`copy_charge_species_to_particle`	Fills the per-particle `charge` field with the species charges.
`sum_charges`	Total charge, sum of squared charges and number of atoms, from the species charges. Needed by Ewald and PPPM.
`sum_charges_pc`	Same, from the per-particle `charge` field.

Migrating from older inputs¶

Several operators were renamed or replaced. Renamed operators keep working under their old name and print a deprecation warning once. Removed operators stop the run with a message naming their replacement.

Old name	Now	Notes
`coul_wolf_pair_*` (pair style `coul_wolf_pair`)	`coul_wolf_*` (`coul_wolf`)	Renamed, same parameters.
`reaction_field_*` (pair style `reaction_field`)	`coul_rf_*` (`coul_rf`)	Renamed, same parameters.
`reaction_field` (per-atom charge operator)	`coulombic_rf`	Same parameters. The `compute_virial` slot is gone: the virial is computed with the energy.
`copy_charge_specy_to_particle`	`copy_charge_species_to_particle`	Renamed.
`coul_wolf_pc`	`coulombic_wolf`	Removed. Same parameters; the self energy is now included.
`coul_dsf_pc`	`coulombic_dsf`	Removed. Same parameters; the self energy is now included.
`coul_wolf_self`	(included in `coulombic_wolf`)	Removed. With the `coul_wolf` pair style, use `coulombic_wolf_self` with `per_atom_charge: false`.
`ewald_init`	`coulombic_ewald_init` or `coulombic_pppm_init`	Removed. New parameters, see Long range.
`ewald_short_range_pc`, `ewald_short_range_*`	`coulombic_ewald_short_range`	Removed. Parameters come from the initialization operator.
`ewald_long_range`, `ewald_long_range_pc`	`coulombic_ewald_long_range` or `coulombic_pppm`	Removed.
`ewald_potential_energy_shift`	(included)	Removed. The self and background energies are part of the long range operators.