Analytic gradient for the state-specific multireference Brillouin-Wigner coupled cluster method.

Abstract:
The aim of this project is to formulate and implement analytic gradient for the state specific multireference Brillouin-Wigner coupled cluster method with single and double excitations (MR BWCCSD), in a form suitable for routine applications by a typical user of quantum chemistry programs. Subsequently we plan to apply this technique to system of interest to chemists, which require multireference description. The development of the MR BWCCSD method has been already subject of our previous research; it turns out that the method yields reliable numbers for systems where single-reference description fails, while its computational cost is only by a factor proportional to the number of reference configurations larger than for the standard CCSD method. In order to be able to perform geometry optimizations and vibrational frequency calculations by this method, it will be necessary to implement for it the analytic first derivatives of the energy.