Potential energy surface of the electron excited states in the state-specific multi-reference coupled cluster theory. Hydrogen fluoride dissociation

The recently proposed state-specific multi-reference coupled cluster (SSMRCC) theory with the complete active space (CAS) reference has been tested in calculations of the potential energy surfaces of electronically-excited states. The algorithm for the method is derived using the computer-based automated approach for generating the coupled cluster diagrams and the energy and amplitude equations. The active space for the calculation of the target state is constructed using natural orbital expansion obtained from the configuration interaction method with single and double excitations (CISD) one-particle density matrix. The spin-orbitals for the CASCC calculation are obtained using the complete active space self consistent field (CASSCF) method. The numerical example shown concerns several electronically excited states with different spatial and spin symmetries of the hydrogen fluoride molecule. The calculations are performed at several internuclear distances and compared with the results obtained using other methods such as the full configuration interaction (FCI) method, the equation-of-motion (EOM) method, and the multi-reference perturbation theory (MRPT). The comparison shows that the CASCC results are better than the EOM and MRPT results.