Accurate structural data for the two rotamers of ground-state HOCO are determined in a series of CI calculations which use a localized orbital representation. The trans rotamer is found to be 3.3±0.5 kcal/mole lower in energy than the cis. In contrast, the Hatree-Fock approximation does not correctly predict the relative stability of these rotamers, the cis isomer more stable than the trans by 0.15±0.1 kcal/mole. A general method which permits a complete and economical analysis of correlation contributions to any desired molecular property is presented. It is based on a partitioning of the Cl matrix according to a well-defined subspaces and type of excitations. Its application to the energy difference between cis- and trans-HOCO shows that the predicted stability of the trans isomer is due to correlation effects uhich shift charge into the open-shell orbital localized on carbon. Electrons residing in lone-pair orbitals are more mobile than electrons in bonding orbitals. This mobility is enhanced when the unfilled and lone-pair orbitals are oriented trans to one another. It is this enhancement which is responsible for stabilization of trans-HOCO. Dipole moments and a population analysis of the CI wavefunction are also used to assist in the identification of correlation effects which make different contributions in the two isomers. Transition states for the rotational and in-plane barriers for interconversion are examined. The predicted rotational barrier is 10.1 kcal/mol above the trans isomer and 6.7 kcal/mol above the cis. The lowest planar interconversion barrier is estimated 34 kcal/mol above the trans isometric energy. © 1985.