Towards a new correction method for the basis set superposition error: Application to the ammonia dimer

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Abstract
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 g a m e s s. 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.