Molecule Type Force Field (MTFF)

A new method is implemented in DFF. The key point of this method is to identify force fields by molecule types instead of atom types as in traditional force field methods.

Molecule types are identified by their constituent molecular fragments. Any molecules containing the same set of fragments, regardless of their sizes (molecular weights) or topological structures (linear, branched, cyclic, number of repeat units or residues, etc.), are the same molecule type.

Since a molecule type is identified by its fragments, an MTFF is derived by using the fragments as the training set. The number of fragments is usually small, so the MTFF can be obtained readily by regression of QM data for those fragments. Least-squares fitting, one of the most common methods, is used in DFF.

For a simulation model that contains multiple molecule types, a force field is constructed by combining the molecule-type force fields. Only intermolecular interactions need to be described, for which the common combination rules for Lennard-Jones and electrostatic interactions are applied.

The same fragments can appear in different molecule types; therefore, the QM data of fragments are saved in a database. For any molecule types that have been parameterized, their force fields are also saved in the database. Because the database contains both fragment data and molecule-type force fields derived from those fragments, it is called the Molecule Fragment Derived Force Field database (MFDFF.db).