Effects of CCSD effective Hamiltonian on Electronic Spectra of noble-gas atoms

It is one of the central topics in theoretical chemistry to develop numerical methods such as the transcorrelated method that inject electron correlations into the Hamiltonian by similarity transformations. In the transcorrelated theory, electron correlations are incorporated by the Baker-Campbell-Hausdorff (BCH) similarity transformation of the Hamiltonian using exponential operators. The exponential operators are also used in the coupled cluster theory (CC) and can be used to act the BCH transformation on the Hamiltonian. However, when the similarity-transformed Hamiltonian by the CC amplitude is used with multi-reference electronic structure theory, it is not known well to the best of the author’s knowledge how it affects prediction of the electronic spectra which is experimentally important for atoms and molecules. In this study, focusing on the electronic spectra of noble-gas atoms, we investigate how an electronic structure is affected by the similarity-transformed Hamiltonian with the CC amplitude combined with multi-reference electronic structure theory. The complete active space (CAS) configuration interaction method using the BCH-similaritytransformed Hamiltonians (the BCH-CASCI method) regardless of the size of the CAS, in many cases the calculated values are closer than the CASCI method to the experimental values of ionization potentials in the case of an Ar atom. The one-electron spectra obtained by the BCH-CASCI method agreed generally with the CCSD method in an Ar atom compared with those obtained by the CASCI method. However, in a Xe atom which is a heavy element, the one-electron spectra which are obtained by this method are affected by non-Hermitian operators.