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 fij (or Fij) 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.