We review a recent development in a rigorous non-Born-Oppenheimer method, i.e., nuclear orbital plus molecular orbital (NOMO) method, which determines the nuclear and electronic wave functions simultaneously. The NOMO theory is an exact theory for the non-BO problem in principle; for example, full-configuration interaction formulation for a complete configuration space. Hartree-Fock (HF) equations for NOs and MOs are derived for practical calculations. The usage of Gaussian basis functions for NOs is discussed. We formulate the elimination of translational and rotational contaminations in the NOMO method. Further, many-body effect such as nucleus-nucleus (n-n), nucleus-electron (n-e), and electron-electron (e-e) correlations are investigated by applying the second-order Møller-Plesset perturbation theory to the NOMO method. The excited-state theories such as configuration interaction and generator coordinate methods are examined to describe not only electronic but also vibrational excited states.
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