A force field expressed in terms of bond-stretching, angle-bending, torsional and other displacements directly connected to the structural parameters of the molecule: \[V=\frac{1}{2}\ \sum k_{ij}\ R_{i}\ R_{j}\] or \[V=\frac{1}{2}\ \sum K_{ij}\ R_{i}\ R_{j}\] where k (or K) are the force constants and R the internal valence coordinates.