Coupled cluster Green's function method: Working equations and applications

Detailed working equations are derived for the ionization part of the single‐particle Green's function within the coupled cluster Green's function ( CCGF ) framework. The CCGF method is applied to the calculation of vertical ionization potentials ( IP s) of a number of small molecules, notably, HF, N 2 , CO, F 2 , CS, C 2 H 4 , H 2 O, and H 2 CO. The results for the outer‐valence IPs, with an average error of 0.12 eV, compare favorably to third‐order equation‐of‐motion calculations within the same basis set (average error 0.28 eV) and outer‐valence GF ( OVGF ) values taken from the literature (average error 0.17 eV). Ground‐state properties that derive from the CCGF are compared to expectation values obtained in the related normal coupled cluster methods ( NCCM ) approximation from a formal point of view. Correlation energies obtained in CCGF are compared to CCSD results for the above series of molecules and, in addition, the so‐called true correlation energy density as obtained from the CCGF is compared to the result from an accurate MR–CI calculation for a highly correlated system: the HF molecule at large internuclear separation.