New Approach to Electronic Structure Calculations for Diatomic Molecules: Application to F2 and Cl2

A new method is described for the calculation of diatomic molecule wavefunctions. Two‐center molecular orbitals (MO's) are used directly, and all integrals are evaluated by strictly numerical means. By avoiding the use of analytic basis functions, the calculation of accurate configuration interaction (CI) wavefunctions requires the computation of a vastly smaller number of molecular integrals. Using this approach it is possible to compute MO CI wavefunctions which are exactly equivalent to complete, high accuracy valence bond (VB) wavefunctions. Other ways of using the present approach to compute wavefunctions of beyond molecular Hartree–Fock (HF) accuracy are discussed. For the F₂ molecule, using accurate (within 0.00001 hartree of the exact HF energy) HF atomic orbitals, complete valence bond wavefunctions are computed for nine internuclear separations, and a dissociation energy of 0.32 eV is obtained. Experimental values of the F₂ dissociation energy range between 1.39 and 1.86 eV. Similarly, high accuracy VB wavefunctions were obtained for Cl₂, yielding a dissociation energy to 0.71 eV, to be compared with the experimental value 2.48 eV. For F₂ more complicated CI wavefunctions, starting from HFAO's, were computed. These wavefunctions added additional $d$ or $p$ functions on each center and included up to 318 configurations and 1438 distinct determinants. The calculations yielded total energies substantially below those previously obtained for the F₂ molecule. However, the largest calculated value of $D_e$ for F₂ was 0.80 eV. We conclude that it is very difficult to obtain a quantitative ab initio dissociation energy for F₂ starting from symmetry MO's which are linear combinations of HFAO's.