A force field expressed in terms of \(3\ N-6\) 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 @F02481@ 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.