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Coulomb (reaction field)

Description

The coul_rf_compute_force operator calculates the reaction-field (RF) electrostatic pair potential assuming a dielectric continuum beyond the cutoff:

For \(r \le r_c\), $$ E(r) = \frac{1}{4\pi \varepsilon_0}\, q_i q_j \left[ \frac{1}{r} + k_{\mathrm{rf}} r^2 - C_{\mathrm{rf}} \right], $$ with $$ k_{\mathrm{rf}} = \frac{\varepsilon_{\mathrm{rf}} - 1}{2\varepsilon_{\mathrm{rf}} + 1}\,\frac{1}{r_c^3}, \qquad C_{\mathrm{rf}} = \frac{1}{r_c} + k_{\mathrm{rf}} r_c^2, $$ so that \(E(r_c)=0\). For \(r>r_c\), \(E=0\). The charges are the species charges. For per-atom charges, use coulombic_rf, see Short range electrostatics.

Note

This style was named reaction_field before. The old operator names (reaction_field_compute_force...) and the potential name in compute_force_pair_multimat are still accepted, with a deprecation warning.

Parameter Units Description
epsilon — Dielectric constant \(\varepsilon_{\mathrm{rf}}\) of the surrounding continuum
rc distance Cutoff radius \(r_c\) of the reaction field (use the same value as rcut)

YAML syntax

coul_rf_compute_force:
  rcut: VALUE UNITS
  parameters: { epsilon: VALUE , rc: VALUE UNITS }
  • VALUE = Physical value of the intended parameter.
  • UNITS = Units of the provided value that will be passed to the conversion helper for internal units conversion.

Usage examples

Systems with a single atomic specy

# Default variant
coul_rf_compute_force:
  parameters: { epsilon: 78.5 , rc: 12.0 ang }  # e.g., water at room T
  rcut: 12.0 ang

# Symetric variant
coul_rf_compute_force_symetric:
  parameters: { epsilon: 78.5 , rc: 12.0 ang }
  rcut: 12.0 ang

Systems with multiple atomic species

coul_rf_multi_force:
  rcut: 12.0 ang
  common_parameters: { epsilon: 78.5 , rc: 12.0 ang }
  parameters:
    - { type_a: Na , type_b: Cl , rcut: 12.0 ang , parameters: { } }
    - { type_a: Na , type_b: Na , rcut: 12.0 ang , parameters: { } }