Rigorous non-Born-Oppenheimer theory: Combination of explicitly correlated Gaussian method and nuclear orbital plus molecular orbital theory

The present study proposes a rigorous non-Born-Oppenheimer theory combining between the explicitly correlated Gaussian (ECG) method and the nuclear orbital plus molecular orbital (NOMO) method. The new method, called ECG-NOMO, adopts the ECG functions between the electronic and nuclear coordinates and, therefore, is capable of describing the nucleus-electron correlation effect accurately. The basic formalism of the ECG-NOMO method is close to the NOMO method, which starts with the Hartree-Fock type equations for NOs and MOs. The present method requires more computational cost than the original NOMO method. However, its cost is significantly smaller than that of the ECG method. The numerical tests was performed for hydrogen-like atoms (H-Ne⁹⁺) and dihydrogen cations (H₂⁺, D₂⁺ and T₂ ⁺), and clarified that the ECG-NOMO method shows the sufficient accuracy.