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

Accurate SCF and MP2 quartic property hypersurfaces have been computed for the energy, quadrupole moment and polarizability tensor of ethylene to obta in zero-point vibrational corrections to the properties. Coupled with accur ate 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 b een made of these properties that incorporate the effects of vibrational av eraging. The effect of deuterium substitution on the properties was investi gated, and the frequency dependence of the polarizability tensor was studie d also. Careful attention has been paid to a critical comparison between th ese theoretical estimates and experimental measurements, and agreement betw een the two is shown to he exceptionally good. In particular, it is possibl e to resolve the disagreement between recent theoretical calculations and e xperimental measurements of the Cotton-Mouton constant. The results focus a ttention on both the general utility of the present method, and the necessi ty to allow for the effects of zero-point vibrational averaging when compar ing theory with experiment, or even when comparing different theoretical re sults with one another using experiment as a benchmark.