general force field

Acronym: gff

A force field expressed in terms of 3 N − 6 basis coordinates:
V = 1 2 ∑ f i j S i S j
V = 1 2 ∑ F i j S i S j
where f i j (or F i j) are force constants and the basis coordinates S (or sometimes s) may be internal symmetry coordinates, local symmetry coordinates or any others suitable to the problem, but the number of the coordinates has to be reduced to 3 N − 6 (3 N − 5 for linear molecules), N being the number of atoms in the molecule.
Source:
PAC, 1978, 50, 1707 (Definition and symbolism of molecular force constants) on page 1709