Abstract:

Potential energy curves of BH(1Σ ) and AlH(1Σ ) molecules in their ground electronic states were calculated at RQCISD(T), UQCISD(T), RCCSD(T), UCCSD(T) and CASSCF (with four active electrons in five active orbitals) theoretical levels. Basis sets examined were 6-311＋ G(3df,2p),G3 large ,aug-cc-pVDZ and aug-cc-pVTZ and the bond lengths tested were up to 1.0 nm. Potential energy curves of BF(1Σ ) and AlF(1Σ ) were also examined at MP4/aug-cc-pVDZ level. It was shown that the MPn (n=2－ 4) curves for BH, AlH and BF were seriously in error. In accordance with CASSCF wavefunctions, the single Hartree-Fock reference state was insufficient to describe the electronic structure of those molecules. In this case, the Hartree-Fock energy gaps between the LUMO and HOMO orbitals for these molecules were decreased when the bond distances were increased. By examination of the molecular orbital integrals, it was found that the interaction (integral values) between HOMO and LUMO orbitals increased greatly. As in the case of second-order MP perturbation calculations, the "stronger interaction" and the "smaller energy gap" resulted in too large correlation energy corrections and therefore led to serious errors in the total electronic energy. This predicted clearly that the MPn extrapolation technique in the widely used G1,G2,G3 and CBS series accurate model chemistry was not reliable in general if the molecule is not at its equilibrium geometry, as what in the study of chemical reaction paths. However, QCISD(T) calculations reflected approximately the correct potential energy curves and more reliable dissociation limits. The maximum error,40.6 kJ• mol－ 1,of dissociation energy at the bond length of 1.0 nm was found in AlH molecule by the G2 basis sets 6-311＋ G(3df,2p).The CCSD(T) was also found less reliable than the QCISD(T) method in describing the dissociation energies. The same error was 101.9 kJ• mol－ 1.All of the results in this work suggested the non-extrapolation G2(QCI) be more reliable a method in the energy calculations of non-equilibrium structure of molecules and be more valuable in further developments and modifications to the methods of accurate model chemistry.

Key words: Single reference state, Mller-Plesset perturbation, Extrapolation, Model chemistry, Non-equilibrium structure, Unreliable