ON THE USE OF SMALL D-SPACES IN SCF AND CI CALCULATIONS ON TRANSITION-METALS

The possibility to use atomic basis sets with only two d-functions des cribing the metal atom has been investigated, both at the SCF and the CI levels, for the Ni and Pd atoms, their mono-hydrides and their mono -oxides. Using a general contraction scheme, two atomic d-orbitals, co rresponding to two of the atomic configurations s(2)d(8), s(1)d(9) and s(0)d(10) were used as the two d-type basis functions. A surprisingly large part of the correlation effect on both atomic excitation energi es and on the equilibrium distance and dissociation energies of the hy drides and the oxides was obtained using these small basis sets. It is argued that the reason for this is important ''near-degeneracy'' effe cts involving different atomic d-configurations and not dynamic correl ation effects.