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 sim- ilarity transformations. In the transcorrelated theory, electron correlations are incorporated by the Baker-Campbell-Hausdorff (BCH) similarity transformation of the Hamiltonian us- ing 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 though some researches have been conducted 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-similarity-transformed Hamiltonians (the BCH-CASCI method) in the case of light elements of the noble-gas atoms, in many cases the calculated values achieves chemi- cal accuracy and are closer than the CASCI method to the experimental values of ionization potentials. The one-electron spectra obtained by the BCH-CASCI method is also improved compared to the CASCI method from the point of view of the relationship between orbital energies and ionization potentials. We also show the results of these methods about a heavy element such as Xe atom.