Coulomb (DSF)¶

Description¶

The coul_dsf_compute_force operator calculates the damped shifted force Coulomb pair potential (Fennell and Gezelter):

\[ E(r) = \frac{q_i q_j}{4\pi\varepsilon_0}\left[\frac{\operatorname{erfc}(\alpha r)}{r} - \frac{\operatorname{erfc}(\alpha r_c)}{r_c} + \left(\frac{\operatorname{erfc}(\alpha r_c)}{r_c^2} + \frac{2\alpha}{\sqrt{\pi}}\frac{e^{-\alpha^2 r_c^2}}{r_c}\right)(r - r_c)\right] \quad \text{for } r < r_c \]

with damping parameter \(\alpha\) and cutoff \(r_c\). Both the energy and the force vanish at \(r_c\). The charges are the species charges.

\(\operatorname{erfc}(\alpha r)\) uses the Abramowitz–Stegun approximation, so \(E(r_c)\) is about \(1.8\times10^{-7}\) eV per unit charge product instead of 0. The pair template subtracts this value from every pair: energies differ from coulombic_dsf (which does not shift) by this constant per pair, forces are identical.

The self energy of each particle is not included: add the coulombic_dsf_self operator with per_atom_charge: false. For per-atom charges, use coulombic_dsf. Both are described in Short range electrostatics.

Parameter	Units	Description
`alpha`	1/distance	DSF damping parameter \(\alpha\)
`rc`	distance	Cutoff radius \(r_c\) of the DSF shift (use the same value as `rcut`)
`rcut`	distance	Cutoff radius of the pair potential

YAML syntax¶

coul_dsf_compute_force:
  rcut: VALUE UNITS
  parameters: { alpha: VALUE UNITS^-1 , 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_dsf_compute_force:
  parameters: { alpha: 0.20 ang^-1 , rc: 10.0 ang }
  rcut: 10.0 ang

# Symetric variant
coul_dsf_compute_force_symetric:
  parameters: { alpha: 0.20 ang^-1 , rc: 10.0 ang }
  rcut: 10.0 ang

# Self energy (thermodynamic output only)
coulombic_dsf_self:
  parameters: { alpha: 0.20 ang^-1 , rc: 10.0 ang }
  per_atom_charge: false

Systems with multiple atomic species

coul_dsf_multi_force:
  rcut: 10.0 ang
  common_parameters: { alpha: 0.20 ang^-1 , rc: 10.0 ang }
  parameters:
    - { type_a: Na , type_b: Cl , rcut: 10.0 ang , parameters: { } }
    - { type_a: Na , type_b: Na , rcut: 10.0 ang , parameters: { } }