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: $R_e{\text{=1.463\hspace{0.17em}Å, ω}}_e\text{= 1270\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ , and $B_e\text{= 1.365\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . 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.