CALCULATION OF BARRIERS TO PROTON-TRANSFER USING MULTICONFIGURATION SELF-CONSISTENT-FIELD METHODS .1. EFFECTS OF LOCALIZATION

The usefulness of multiconfiguration self-consistent-field (MCSCF) calculations in computing correlated proton transfer potentials is investigated for the systems HF2-, H7N2+, H3O2-, and H5O2+. In deciding whether to include particular molecular orbitals, it is important to consider the balance of electron density between the donor and acceptor groups and the interactions that are incorporated in the orbitals. Only orbitals which have the proper symmetry to interact with the transferring hydrogen need be included in the MCSCF active space. Reasonable transfer barriers are obtained when the orbitals are balanced and only interactions relevant to the transfer process are allowed in the MCSCF active space. Equivalent barriers are determined, but the criteria are more easily met, if the canonical molecular orbitals are first subjected to a localization. Only the two localized molecular orbitals that contain the F, N, or O interaction with the transferring hydrogen are needed, which reduces the difficulty of eliminating unproductive interactions. In addition, the localization allows additional virtual orbitals to be included without producing a undesirable correlation.