Density Matrix Study of Atomic Ground and Excited States. I. Beryllium Ground State

Abstract

Various configuration‐interaction wavefunctions for the Be ground state built up from Slater‐type orbitals are reported. Their 1‐ and 2‐matrices are examined in detail and compared with those previously reported in the literature. It is shown how the quality of a basis set and the omission of just a few configurations noticeably alter the density matrix structure. Further, it is pointed out that the occupation numbers of some important natural states are in general quite sensitive to the goodness of the wavefunction. In the choice of configurations to be included in a truncated CI expansion over an extended basis set by performing stepwise full CI expansions over small basis sets, the role played by singly excited configurations is emphasized. The use of density matrices as a global representation of the wavefunction is also stressed.