General-order single- and multi-reference configuration interaction and coupled-cluster theory: Symmetric dissociation of water

We present a determinant-based method used to formulate many-body wave functions and energy expectation values of any quantum chemical model which can be written in terms of second-quantized operators. The method is used to apply single- and multi-reference configuration interaction and coupled-cluster theories, with restricted Hartree–Fock (RHF), unrestricted Hartree–Fock (UHF), and complete-active-space self-consistent-field (CASSCF) orbitals, to the symmetric dissociation of water. Results from unrestricted state-selective multi-reference coupled-cluster theory are presented for the first time.