Double-ζ SCF MO Calculation of the Ground and Some Excited States of N2

Self-consistent field wave functions for the ground and certain ionized and excited states of the nitrogen molecule, evaluated at the ground state internuclear distance, 1.094 A, are obtained in the molecular orbital (MO) form. The MO's are represented in the usual linear combination of atomic orbitals (LCAO-MO) form, but an expanded basis set is used which contains two each of 2s, 2pσ, 2pπ, and 2pπ̄ Slater-type atomic orbitals of differing nuclear charges. All electronic interactions are properly included and calculated without approximation. The computed total energy of the ground state is —108.785 a.u., which is 21.8 ev above the experimental value but 5.8 ev better than a previous result in which strictly atomic (Slater) orbitals were used in the conventional LCAO form. Excitation and ionization energies obtained by direct calculation of the respective states involved are in reasonably good agreement with experiment, though no better than those same quantities calculated from ground-state orbitals in the usual manner. The wave functions for the various states are analyzed and implications drawn regarding the changes wrought in atomic orbitals by bond formation; relevance to the semiempirical interpretation of quadrupole coupling constants is emphasized. Some conjecture about the correlation energy suggests that the ground-state energy obtained may be only a few electron volts above the true molecular Hartree-Fock energy.