Perturbation Expansion Theory Corrected From Basis Set Superposition Error II. Charge-transfer, pair correlation and dispersion terms

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The second order perturbation theory based on the locally projected molecular orbitals is devel- oped. A few test calculations with cc-pVDZ and aug-cc-pVDZ basis sets are carried out for the dimers, (H2O)2 and (HF)2. The charge transfer terms remove the de.ciency of the locally projected self-consistent .eld method for molecular interaction (LP SCF MO MI), and the potential energy curves calculated with aug-cc-pVDZ are very close to the corresponding curves of the counterpoise corrected SCF energy. Only after adding the spin-exchanged dispersion type to the dispersion and intra-molecular pair correlation terms, the calculated potential energy curves become close to those of the couterpoise corrected second order Moller-Plesset (MP2). Pragmatic approaches for reducing the in.uence of the basis set superposition error are proposed.