Ab initio study of the X, ã, Ã and B states of formaldehyde

Abstract

The optimum geometrical structures of formaldehyde in the ground state (X¹A₁), the (n, π*) states (ã³A″ and ùA″), and the lowest Rydberg state (3s, B¹B₂) have been obtained using SCF and GVB energies and a few basis sets built on the usual double zeta (DZ) set. In the quasi-Newton optimization process an approximate force constant (Hessian) matrix is obtained for no extra cost and, by comparison with other methods, it is shown that the approximate Hessian yields reasonable vibrational wavenumbers in most cases. The out-of-plane bending potential is studied in the ã and à states, and the excitation energies from the ground state to the ã, à and B states are compared with experimental values.