Exponential-6¶

Description¶

The exp6_compute_force operator calculates the Buckingham exponential-6 pair potential with an additional short-range repulsive wall:

\[ E(r) = A\,e^{-Br} - \frac{C}{r^{6}} + D\left(\frac{12}{B\,r}\right)^{12} \quad \text{for} \quad r<r_c \]

The last term is a steep \(r^{-12}\) wall that prevents the collapse of the \(-C/r^6\) term at very short distances (the "Buckingham catastrophe"). Set \(D=0\) to recover the plain exponential-6 form.

Parameter	Units	Description
`A`	energy	Amplitude of the exponential repulsion
`B`	1/distance	Inverse range of the exponential
`C`	energy·distance\(^6\)	Dispersion coefficient
`D`	energy	Amplitude of the short-range wall
`rcut`	distance	Cutoff radius

YAML syntax¶

exp6_compute_force:
  rcut: VALUE UNITS
  parameters: { A: VALUE UNITS , B: VALUE UNITS , C: VALUE UNITS , D: 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
exp6_compute_force:
  parameters: { A: 107023.9 Da*kcal/g , B: 3.6405 1/ang , C: 554.01 Da*kcal*ang^6/g , D: 5.0e-5 Da*kcal/g }
  rcut: 12.5 ang

# Symetric variant
exp6_compute_force_symetric:
  parameters: { A: 107023.9 Da*kcal/g , B: 3.6405 1/ang , C: 554.01 Da*kcal*ang^6/g , D: 5.0e-5 Da*kcal/g }
  rcut: 12.5 ang

Systems with multiple atomic species

exp6_multi_force:
  rcut: 12.5 ang
  common_parameters: { A: 107023.9 Da*kcal/g , B: 3.6405 1/ang , C: 554.01 Da*kcal*ang^6/g , D: 5.0e-5 Da*kcal/g }
  parameters:
    - { type_a: C , type_b: C , rcut: 12.5 ang , parameters: { A: 107023.9 Da*kcal/g , B: 3.6405 1/ang ,     C: 554.01 Da*kcal*ang^6/g ,      D: 5.0e-5 Da*kcal/g } }
    - { type_a: C , type_b: N , rcut: 12.5 ang , parameters: { A: 37111.29 Da*kcal/g , B: 3.46350030 1/ang , C: 484.2991571 Da*kcal*ang^6/g , D: 5.0e-5 Da*kcal/g } }

Note

Da*kcal/g is kcal/mol expressed per particle (1 Da = 1 g/mol).