An ab initio study of vibrational corrections to the electrical properties of ethylene

Accurate SCF and MP2 quartic property hypersurfaces have been computed for the energy, quadrupole moment and polarizability tensor of ethylene to obtain zero-point vibrational corrections to the properties. Coupled with accurate electrical properties computed at a high level correlated r(e) geometry, using a range of correlated methods, especially ED and BD(T), along with a number of purpose-built polarized basis sets, definitive estimates have been made of these properties that incorporate the effects of vibrational averaging. The effect of deuterium substitution on the properties was investigated, and the frequency dependence of the polarizability tensor was studied also. Careful attention has been paid to a critical comparison between these theoretical estimates and experimental measurements, and agreement between the two is shown to he exceptionally good. In particular, it is possible to resolve the disagreement between recent theoretical calculations and experimental measurements of the Cotton-Mouton constant. The results focus attention on both the general utility of the present method, and the necessity to allow for the effects of zero-point vibrational averaging when comparing theory with experiment, or even when comparing different theoretical results with one another using experiment as a benchmark.