Ab initio quantum chemical calculation of the pair potentials of superconductors

Ab initio quantum chemical calculation on model clusters, Al14, Pb14, V16, Nb16, Sn14, In18 and Hg18, are presented to estimate the pairing potentials of superconductivity. The total energies of the clusters for the restricted Hartree-Fock (RHF) type and unrestricted Hartee-Fock (UHF) type singlet ground states and the triplet state are calculated by the hybrid DFT method of Gaussian G03 program. The energy differences between these states are correlated to the superconducting energy gap 2Δ. By a slight modification of the geometry of the clusters from their positions in the parent crystalline lattice, reasonable 2Δ values are obtained. We regard the UHF type singlet state (mixed singlet state, MS) as to represent the antiferromagnetic metallic state and it is the state for superconductivity. Comparing the calculated results with other metal clusters we propose a general rule for appearance of superconductivity in elementary metals.