TOWARDS A NEW CORRECTION METHOD FOR THE BASIS-SET SUPERPOSITION ERROR - APPLICATION TO THE AMMONIA DIMER

In this paper, we investigate a new method for correcting the basis set superposition error (BSSE) in ab initio quantum calculations of hydrogen-bonded (H-bonded) molecular complexes. In our scheme, the Hartree-Fock molecular orbitals (MOs) are first localized. The localized MOs (LMOs) are then separately attributed to each of the component or fragment molecules. Considering each LMO assigned to a specific fragment molecule, we set to a zero value the LMO coefficients relative to the AOs belonging to all the other partner molecules or molecular fragments. After purification of the "off-fragment" coefficients, the LMOs are then reorthonormalized. The resulting wave function constitutes a first level of approximation to a BSSE-corrected wave function. An iteration procedure is then implemented, comprising the following steps: HF MOs; localization; fragment attribution; off-fragment purification; reorthormalization; new Fock matrix; diagonalization of the Fock matrix; HF-type MOs; localization and so on. The converged wave function satisfies a self-consistent equation. The scheme can be extended to MCSCF wave functions. The MCSCF MOs are localized, then off-fragment LMOs components are eliminated. The resulting LMOs are reorthonormalized to generate a MO basis for a CI computation. This BSSE correction scheme is implemented within GAMESS. Applications to the ammonia dimer are described in this and the accompanying paper. We find that linear H-bonded geometries are artificially favored by the BSSE. © 1995 American Institute of Physics.