COORDINATE TRANSFORMATIONS OF CUBIC FORCE-CONSTANTS AND TRANSFERABILITY OF ANHARMONIC-FORCE CONSTANTS IN INTERNAL COORDINATES

A general approach to transforming cubic anharmonic force constants from the Cartesian coordinates to an arbitrary set of nondegenerate internal coordinates is presented and a computer program performing the transformation is described. It allows one to study the transferability of the cubic anharmonic force constants between similar molecules. The harmonic and anharmonic force constants are calculated at the Hartree-Fock level for the C2H6-nF(n) series. The diagonal cubic force constants for the bond stretching are found to be transferable to the same extent as the quadratic (harmonic) ones. The force constants for the bendings are not directly transferable. The present approach makes it possible to construct anharmonic force fields by fitting parametric functions to the ab initio anharmonic force constants in the internal coordinates.