Ab initiocalculation of the lowest ionization potential and transition energies of methane

The lowest ionization potential and transition energies of methane are calculated by a second-order perturbation expansion, using a limited basis of gaussian-type orbitals. An attempt is made to rationalize the choice of basis atomic orbitals. This procedure yields 13·8 eV for the first ionization potential and 10·4 eV for the lowest allowed transition provided a diffuse molecular orbital is added to describe the excited state (the experimental values are 13·6 and 9·7 eV). These two values include contributions of -1·0 and -0·70 eV, respectively, from an approximate optimization of the basis atomic orbitals and of +0·50 and +0·25 eV from electron correlation.