Correlation Energy Calculation for the 1Σg+ Ground State of the Nitrogen Molecule

Starting from a molecular Hartree—Fock wavefunction an extensive configuration-interaction calculation (201 configurations) has been carried out for the 1Σg+ ground state of N2. The matrix elements of the electrostatic Hamiltonian were calculated automatically. Both Brillouin—Wigner and Rayleigh—Schrödinger perturbation expansions, carried to a sufficient order, are shown to be good tools to find the lowest eigenvalue and eigenvector of very large matrices. Fifty percent of correlation energy is attained by this exact treatment. Since the second-order Rayleigh—Schrödinger perturbation expansions are a good approximation to the exact results, it supplies, in addition to a good test for the selecting of configurations, a practical means of evaluating pair correlations. By using this approximation and 3043 configurations, 66% of the correlation energy was found.