COUPLED-CLUSTER METHOD FOR OPEN-SHELL SINGLET-STATES

Open-shell singlet states of molecules must be described by two (spin-orbital) determinants, hence they cannot be calculated with standard single-reference correlated methods. We show that this problem can be solved using elements of the Hilbert space multi-reference coupled-cluster theory. Explicit algebraic equations are introduced and their application is illustrated by studying several excited states of ozone, ketene and diazomethane. Comparison with Fock space multi-reference coupled-cluster solutions is presented.