Hartree—Fock and MCSCF calculations on the cis—trans isomerism of formic acid

Abstract

Ab initio Hartree—Fock and multi configuration self consistent field (MCSCF) calculations on the cis—trans energy difference of formic acid are reported. The basis sets employed range from split valence to better than double zeta with two sets of polarization functions. Three types of error resulting from a Hartree—Fock rigid rotation calculation based on the trans geometry have been evaluated, i.e. the deformation of the cis isomer from its experimental geometry, the incompleteness of the orbital basis and the intrinsic Hartree—Fock errors (“correlation”). All three appear to be of the order of 1 kcal mol −1. A best calculated rigid rotation energy difference of 4.9 ± 0.2 kcal mol −1 is obtained which does not agree satisfactorily with the experimental value, of 4.3 ± 0.2 kcal mol −1 corrected for zero-point energy and deformation energy. The correlation correction considered is equivalent to an oxygen-to-carbon charge redistribution, with respect to the (too polar) Hartree—Fock density in the CO part of the molecule, due to the inclusion of a single π 2 → π *2 doubly excited configuration.