A coupled cluster polarization propagator method applied to CH+

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Abstract
A new approach to the direct evaluation of excitation energies and transition moments from the polarization propagator is presented. The method, which uses a coupled cluster doubles (CCD) reference state within the framework of perturbative propagator methods, is applied to the lowest singlet and triplet excitations in CH+. Comparison of the coupled cluster doubles polarization propagator approximation (CCDPPA) results with experiments and standard perturbative polarization propagator calculations shows that a significant improvement is obtained with a coupled cluster rather than a Rayleigh–Schrödinger reference state: the singlet excitation energy is improved by about 0.5 eV and the triplet instability of the standard second order approach is removed. The radiative lifetime of the v’=0 level of the A 1Π state is estimated to be very close to 800 ns. The improved performance of the coupled cluster propagator method over propagator calculations based on Rayleigh–Schrödinger expansion mainly stems from a enhancement of the correlation coefficients for the lower double excitations.

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