A force field expressed in terms of $3N-6$ basis coordinates: $$V=\frac{1}{2}\sum f_{ij}{S}_i{S}_j$$ $$V=\frac{1}{2}\sum F_{ij}{S}_i{S}_j$$ where $f_{ij}$ (or $F_{ij}$) 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 $3N-6$ ($3N-5$ for linear molecules), $N$ being the number of atoms in the molecule.