Open-shell coupled-cluster theory for a general incomplete model space using eigenvalue-independent partitioning

Multireference coupled-cluster (MRCC) theory using an eigenvalue-independent partitioning (EIP) technique is formulated for a general incomplete model space, taking a valence universal normal ordered wave operator …ifΩ = srmexp(S)s that acts upon the model space P. For the size-extensivity requirement the intermediate normalization is abandoned (i.e. PΩP ≠ P) and hence the closed part of Ω, Ωcl, is not unity. This leads to an implicit equation for the Heff. The EIP structure of the MRCC equation is capable of producing the main as well as the satellite states together.