External Engine
Two external simulation packages, LAMMPS and GROMACS, are included with DFF distributions. These are multi-threaded parallel (MTP) versions released after 2021. Force field validation tasks such as energy minimization, and NVT, NPT, and NVE molecular dynamics can be launched directly from DFF using these programs.
Conversions
Because different software packages use different atom type conventions, DFF translates atom types into the formats recognized by the target program. This translation is a direct one-to-one mapping between DFF atom types and unique string identifiers used by the external software.
Since potential functions may be implemented differently from their published forms, some parameters must also be converted into the closest functional equivalents before export.
Dihedral in DREIDING
The dihedral potential in DREIDING is expressed as:
In LAMMPS, the closest function is the CHARMM dihedral_style:
The conversion between the two forms is given by:
Thus, the force constant remains unchanged (Eq. 3), the periodicity is preserved (Eq. 4), and the phase shift is adjusted by
Out-of-Plane in DREIDING
Two different functions are used for the out-of-plane (improper) potential in DREIDING.
For non-planar structures (
For planar structures (
LAMMPS uses a harmonic function (class2 improper style):
To achieve equivalence between the two functional forms, the following conversions are applied:
If
If
In summary, the reference angle remains unchanged (Eq. 9). For non-planar cases, the force constant is scaled by