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Molecular orbital method is a powerful tool to theoretically clarify the properties of molecular systems. The calculation accuracy of the molecular orbital method depends on the number of basis functions expanding the molecular orbital, which is a one-particle wave function of electrons, the treating of electron correlation, and the treating of relativistic effects. Relativistic effects are important mainly for heavy and superheavy elements, sometimes at a qualitative level.
However, relativistic molecular orbital calculation has large computational cost than non-relativistic one. This is a obstacle to clarifying the chemistry of heavy element systems theoretically. The cost of relativistic calculations comes from the facts that the Fock matrix is larger than that of non-relativistic ones, and that integrals of high angular momentum must be calculated.
So, in this talk, I will introduce the research that has been performed as an approach to these problems.