Skip to content

Lennard-Jones + Reaction Field

Description

The ljrf_compute_force operator adds a Lennard-Jones term and a reaction-field electrostatic term:

\[ E(r) = E_{\mathrm{LJ}}(r) + E_{\mathrm{RF}}(r) \]

with

\[ E_{\mathrm{LJ}}(r) = \begin{cases} 4\varepsilon\left[\left(\dfrac{\sigma}{r}\right)^{12} - \left(\dfrac{\sigma}{r}\right)^{6}\right] - E_{\mathrm{shift}} & r \le r_{\mathrm{LJ}} \\ 0 & r > r_{\mathrm{LJ}} \end{cases} \qquad E_{\mathrm{RF}}(r) = \begin{cases} \dfrac{q_i q_j}{4\pi\varepsilon_0}\left[\dfrac{1}{r} + k_{\mathrm{rf}} r^2 - C_{\mathrm{rf}}\right] & r \le r_{\mathrm{rf}} \\ 0 & r > r_{\mathrm{rf}} \end{cases} \]

\(E_{\mathrm{shift}}\) is the Lennard-Jones energy at \(r_{\mathrm{LJ}}\) when shiftlj is true (default), 0 otherwise. \(k_{\mathrm{rf}}\) and \(C_{\mathrm{rf}}\) are defined on the reaction field page. The charges \(q_i\), \(q_j\) are the species charges.

Parameter Units Description
lj_rcut distance Cutoff \(r_{\mathrm{LJ}}\) of the Lennard-Jones term (0 or missing disables it)
lj.epsilon energy Lennard-Jones well depth \(\varepsilon\)
lj.sigma distance Lennard-Jones distance \(\sigma\)
shiftlj — Shift the Lennard-Jones energy to zero at lj_rcut (default true)
rf.epsilon — Dielectric constant of the surrounding continuum
rf.rc distance Cutoff \(r_{\mathrm{rf}}\) of the reaction field
rcut distance Cutoff radius of the pair potential, at least \(\max(r_{\mathrm{LJ}}, r_{\mathrm{rf}})\)

YAML syntax

ljrf_compute_force:
  rcut: VALUE UNITS
  parameters:
    lj_rcut: VALUE UNITS
    lj: { epsilon: VALUE UNITS , sigma: VALUE UNITS }
    shiftlj: BOOL
    rf: { 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
ljrf_compute_force:
  rcut: 12.5 ang
  parameters:
    lj_rcut: 11.0 ang
    lj: { epsilon: 0.01 kcal/mol , sigma: 3.830864488 ang }
    rf: { epsilon: 100. , rc: 12.5 ang }

# Symetric variant
ljrf_compute_force_symetric:
  rcut: 12.5 ang
  parameters:
    lj_rcut: 11.0 ang
    lj: { epsilon: 0.01 kcal/mol , sigma: 3.830864488 ang }
    rf: { epsilon: 100. , rc: 12.5 ang }

Systems with multiple atomic species

ljrf_multi_force:
  rcut: 12.5 ang
  common_parameters: { lj_rcut: 11.0 ang , lj: { epsilon: 0.01 kcal/mol , sigma: 3.83 ang } , rf: { epsilon: 100. , rc: 12.5 ang } }
  parameters:
    - { type_a: C , type_b: C , rcut: 12.5 ang , parameters: { lj_rcut: 11.0 ang , lj: { epsilon: 0.01 kcal/mol , sigma: 3.83 ang } , rf: { epsilon: 100. , rc: 12.5 ang } } }
    - { type_a: C , type_b: N , rcut: 12.5 ang , parameters: { lj_rcut: 11.0 ang , lj: { epsilon: 0.02 kcal/mol , sigma: 3.60 ang } , rf: { epsilon: 100. , rc: 12.5 ang } } }

Note

A rigid molecule variant, ljrf_rigidmol_force, is also available.