Generalized Amber Force Field (gaff)¶
A general set of parameters for small organic molecules to allow simulations of drugs and small molecule ligands in conjugtion with biomolecules is provided by GAFF. Parameters exist for almost all molecules made of C, N, O, H, S, P, F, Cl, Br, and I, and are compatible with the AMBER functional forms.
Typically, GAFF expects partial charges assigned using quantum chemistry (i.e., HF/6-31G* RESP charges or AM1-BCC). The Open Babel implementation can use other partial charges as available, although with lower resulting accuracy.
In general, GAFF is expected to provide accuracy (in terms of geometry and energies) on par or better than the MMFF94 Force Field (mmff94).
If you use GAFF, you should cite the appropriate paper: Wang, J., Wolf, R. M.; Caldwell, J. W.;Kollman, P. A.; Case, D. A. “Development and testing of a general AMBER force field”. Journal of Computational Chemistry, 2004 v. 25, 1157-1174.