An efficient approach for calculating vibrational wave functions and zero-point vibrational corrections to molecular properties of polyatomic molecules

We have recently presented a formalism for calculating zero-point vibration al corrections to molecular properties of polyatomic molecules in which the contribution to the zero-point vibrational correction from the anharmonici ty of the potential is included in the calculations by performing a perturb ation expansion of the vibrational wave function around an effective geomet ry. In this paper we describe an implementation of this approach, focusing on computational aspects such as the definition of normal coordinates at a nonequilibrium geometry and the use of the Eckart frame in order to obtain accurate nonisotropic molecular properties. The formalism allows for a blac k-box evaluation of zero-point vibrational corrections, completed in two su ccessive steps, requiring a total of two molecular Hessians, 6K - 11 molecu lar gradients, and 6K - 11 property evaluations, K being the number of atom s. We apply the approach to the study of a number of electric and magnetic properties-the dipole and quadrupole moments, the static and frequency-depe ndent polarizability, the magnetizability, the rotational g tensor and the nuclear shieldings-of the molecules hydrogen fluoride, water, ammonia, and methane. Particular attention is paid to the importance of electron correla tion and of the importance of the zero-point vibrational corrections for ob taining accurate estimates of molecular properties for a direct comparison with experiment. (C) 2000 American Institute of Physics. [S0021-9606(00)310 05-4].