Complete active space self-consistent field and density functional study of FNO

Abstract

The molecular structure and bond dissociation energy of FNO are studied using complete active space self-consistent field (CASSCF) and density functional methods (DFT), and the results are compared to experimental. Nitrogen–fluorine bond lengths obtained from DFT using the local density approximation are fairly accurate but N–F bond energies are overestimated by more than 40 kcal mol−1. Application of gradient corrections reduces the error in the bond dissociation energy to ∼15 kcal mol−1 but yields bond lengths that are significantly too long. Basis set effects in density functional results are compared to those observed previously in FNO2. The CASSCF method produces accurate bond lengths only with very large (10 in 8 or 12 in 9) active spaces; smaller active spaces yield N–F bond lengths that are short by 0.07 Å or more. The 12 in 9 active space underestimates the bond energy by ∼14 kcal mol−1. The inclusion of certain orbitals in the active space leads to much improved structures and significantly lower absolute energies, but the SCF configuration dominates the wave function and occupation numbers are essentially 2 and 0. From these results, we conclude that the challenge FNO presents to ab initio calculation is due to dynamic electron correlation rather than any multireference character.