Merck molecular force field (MMFF) is a family of chemistry force fields developed by Merck Research Laboratories.
MMFF is not optimized for one use, such as simulating proteins or small molecules, but tries to perform well for a wide range of organic chemistry calculations.
The parameters in the force field have been derived from computational data consisting of approximately 2800 structures spanning a wide range of chemical classes.
[1] A set of molecular structures and the corresponding output of Halgren's MMFF94 implementation is provided at the Computational Chemistry List for validating other MMFF implementations.
[2] One variant of MMFF94 is MMFF94s, which has different out-of-plane bending and dihedral torsion parameters in order to planarize delocalized trigonal nitrogen atoms, e.g. in aniline.