Theory of Molecular Interaction and Its Application to Studies of Molecular Clusters

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To evaluate the molecular interaction energy by the ab initio (first principle) quantum chemical calculations, the error inherent to the basis set expansions has to be removed or to be avoided. The causes of the error are the inconsistency in the approximation for the isolated systems and for the composite system. In the molecular orbital (MO) based theory and in most of the density functional theory (DFT), the one-electron functions (orbitals) are expanded in terms of Gaussian functions. So one of causes in the inconsistency is related with the orbitals used; which can be called the Orbital Basis Inconsistency (OBI). In MO theory, the many-electron wave functions are described by a superposition of electron configurations, or by a linear combination of Slater determinants. Thus, the second cause of the inconsistency is Configuration Basis Inconsistency (CBI). Using the locally projected molecular orbital (LPMO), the 3rd order single excitation perturbation (3rdSPT) and the 2nd order double excitation perturbation theory (2ndDPT) are developed. The theory can avoid the OBI by determining the MO under the local condition and the CBI by restricting the electron configurations in the perturbation theory. The method is tested for many molecular clusters using various basis sets, and the calculated binding energies are compared with the published reference energies. The developed code is sufficiently efficient to study a large water clusters such as (H_2O)_25 with 3rdSPT/aug-cc-pVDZ and (H_2O)_6 with 3rdSPT+2ndDPT/aug-cc-pVTZ.