Atomic orbital basis sets for use with effective core potentials

Basis sets developed for use with effective core potentials describe pseudo -orbitals rather than orbitals. The primitive Gaussian functions and the co ntraction coefficients in the basis set must therefore both describe the va lence region effectively and allow the pseudo-orbital to be small in the co re region. The latter is particularly difficult using Is primitive function s, which have their maxima at the nucleus. Several methods of choosing cont raction coefficients are tried, and it is found that natural orbitals give the best results. The number and optimization of primitive functions are do ne following Dunning's correlation-consistent procedure. Optimization of or bital exponents for larger atoms frequently results in coalescence of adjac ent exponents; use of orbitals with higher principal quantum number is one alternative. Actinide atoms or ions provide the most difficult cases in tha t basis sets must be optimized for valence shells of different radial size simultaneously considering correlation energy and spin-orbit energy. (C) 20 00 John Wiley & Sons, Inc.