Role of Electron Correlation in a Priori Predictions of the Electronic Ground State of BeO

Ab initio wavefunctions including electron correlation have been calculated for the 3II state of BeO. A (4s2p1d) basis set of Slater functions was centered on each atom. The iterative natural orbital method was used to optimize the set of molecular orbitals employed in each 591 configuration first-order wave-function. The 3II energy calculated here is 0.73 eV above the 1Σ+ energy obtained in a comparable calculation. Since near Hartree-Fock calculations result in a 3II energy below the 1Σ+ energy, it seems clear that electron correlation plays a crucial role in the ordering of these states. Predicted spectroscopic constants for the 3II state are: Re=1.463 Å, ωe= 1270 cm−1, and Be= 1.365 cm−1. Natural orbital occupation numbers and coefficients of important configurations in the CI wavefunctions are presented to describe the electronic structure of 3II BeO. First-order calculations (519 configurations) were also carried out for the lowest 3Σ− state of BeO. These calculations confirm our previous SCF prediction that the 3Σ− state is repulsive.