The effect of electron correlation on the energy of the terms arising from a pair of d electrons has been investigated by ab initio methods for the 20-electron isoelectronic series: Ti2+, V3+, Cr4+. Pair correlation energies in the monopole (radial) and quadrupole approach within the truncated multipolar expansion pair function method [K. Jankowski, Can. J. Phys. 48, 2707 (1970)] have been obtained together with the pair functions under consideration. Convergence properties of the correlation energy with respect to expansion of the radial part of the correlation factor have been investigated. Multiplet spacings have been obtained and compared with other results. In addition, an attempt is made to compare our results with those obtained by Mires and co-workers [Phys. Rev. 134, A332 (1964); 185, 16 (1969); A 1, 14 (1970)] in a semiempirical way.
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Jankowski et al. (1972) studied this question.