OPLS torsion¶
Description¶
The opls_torsion type of torsion potential calculates the torsion potential of the OPLS force field:
\[
E = \frac{1}{2}k_1 \left ( 1 + \cos{\phi}\right ) + \frac{1}{2}k_2 \left ( 1 - \cos{2\phi}\right ) + \frac{1}{2}k_3 \left ( 1 + \cos{3\phi}\right ) + e_0
\]
with \(k_1\), \(k_2\), \(k_3\) and \(e_0\) the potential parameters described in the following table.
| Parameter | Units | Description |
|---|---|---|
| \(k_1\) | energy | force constant |
| \(k_2\) | energy | force constant |
| \(k_3\) | energy | force constant |
| \(e_0\) | energy | constant energy offset, optional (default 0) |
YAML syntax¶
molecule_force_field:
potentials_for_torsions:
- types: [STRING, STRING, STRING, STRING]
potential: opls_torsion
parameters:
k1: VALUE UNITS
k2: VALUE UNITS
k3: VALUE UNITS
e0: VALUE UNITS # optional
e0 only shifts the energy. It is useful, e.g., to write a GAFF torsion, a sum of \(\frac{k}{m}(1+\cos(n\phi-\phi_0))\) terms
with \(n \le 3\), in this form with the same energy.
- STRING = Name of the atom type (specie) involved in the torsion angle.
- 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 atom type (specie)
Systems with several atom types (species)