On calculations of correlated wave functions with heavy configurational mixing

We discuss aspects of the theory and computation of wave functions and energies of discrete states of polyelectronic atoms that are represented in zero order by configurations with holes in subshells below the valence subshell. Both in zero order and in the remaining correlation components, such wave functions have particularities stemming from the state‐specific self‐consistent field and the heavy configurational mixing associated with near‐degeneracies and hole‐filling correlations. By referring to a variety of examples from small‐ and large‐scale calculations, it is noted that appropriate penetration into the many‐body problem can provide, in an economic and physically transparent way, reliable interpretations and semi‐ and fully quantitative understanding of issues related to states with inner holes and to cases of near‐degeneracies that result in strongly correlated wave functions. Whenever hole‐filling correlations are allowed, multiple correlations (i.e., beyond single‐ and double‐orbital substitutions in the single reference configuration) acquire increased importance relative to that in ordinary electronic structures. This is demonstrated via large‐scale multiconfigurational Hartree–Fock (MCHF) plus configuration interaction (CI) calculations on the Cl KL3s3p^{6 2}S discrete state, which is the lowest of its symmetry. The calculations incorporated correlations up to selected sextuple orbital excitations from the M shell. MCHF plus CI calculations at the level of quadruple orbital substitutions were also carried out for the Cl KL3s²3p^{5 2}P^o ground state and the excitation energy at this level of calculation was found to be 85,364 cm^?1, in excellent agreement with the experimental value of the fine‐structure‐weighted average, 85,385 cm^?1 (10.59 eV). Within the approximations of the calculation, the hole‐filling triple and quadruple orbital correlations, which, of course, are absent from the ²P^o state, contribute about 1 eV, which is significant. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005