Charge-transfer separability and size-extensivity in the equation-of-motion coupled cluster method: EOM-CCx.

We study the charge-transfer separability (CTS) property of the Fock space (FS) and equation-of-motion (EOM) coupled cluster (CC) methods by analysing the charge-transfer (CT) excitation energy versus the donor-acceptor (D-A) distance. All FS-CC approaches fulfill the CT separability condition which is not the case for the standard EOM-CC approaches. This defect of the EOM-CC scheme can be fixed by slight modification of the H matrix's diagrammatic structure, namely by adding some "dressing" composed of disconnected terms. The latter guarantee CTS of the respective EOM-CC scheme and marginally improve local excitations. The newly proposed variant of the EOM-CCSD approach is termed EOM-CCSDx (size-extensive EOM-CCSD).