Non-Born–Oppenheimer theory for simultaneous determination of vibrational and electronic excited states: ab initio NO+MO/CIS theory

The NO+MO/HF theory has been previously proposed to determine the nuclear and electronic wave functions in the ground state without the Born–Oppenheimer approximation. In this study, we apply the configuration interaction method with single particle excitation operators to the NO+MO/HF wave function. This method, named NO+MO/CIS method, gives not only the electronic excited state but also the vibrational excited state. Numerical applications of the NO+MO/CIS method to H2, D2, T2, and H3+ molecules are performed and confirm its accuracy and feasibility.