subalgebra coupled cluster (SES-CC) formalism to unitary CC formalisms. In analogy to the
standard single-reference SES-CC formalism, its unitary CC extension allows one to include
the dynamical (outside the active space) correlation effects in an SES induced complete
active space (CAS) effective Hamiltonian. In contrast to the standard single-reference SES-
CC theory, the unitary CC approach results in a Hermitian form of the effective Hamiltonian …