The Potential Energy Function For The Ground State And Low-Lying Excited States Of BeF, BeO, AlO Molecules

The research of molecular structure and analytical potential energy function (APEF) is one of the important fields for material science, atomic and molecular physics and quantum chemistry. The APEF can completely determine molecular energy, equilibrium geometry, force constants and spectroscopic parameters. At the same time, it is also the potential function of core movement. The research on the APEF can provide a lot of information for the investigation in the high and new-technology field, such as spaceflight, laser, material, and so on. Therefore, it is necessary to accurately investigate molecular potential energy function on the theoretical level.In this letter, first, the background, significance and the theoretical calculation methods of diatomic molecules for potential function research are introduced. Then, the potential energy curves (PECs) of the ground-state and low-lying excited states for BeF, BeO and AlO molecules are investigated using the complete active space self-consistent-field(CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach. The PECs of these three molecules are fitted to the analytic Murrell-Sorbie (M-S) function, then the spectroscopic parameters (De,ωe,ωeχe,αe, Be and D0) are evaluated.Firstly, the PECs of X~2Σ+, A~2Πr and B~2Σ+ electronic states of BeF radical have been investigated using the CASSCF method followed by the MRCI approach with the basis sets cc-pV5Z for Be and aug-cc-pV6Z for F. Based on the PECs of X~2Σ+, A~2Πr and B~2Σ+ electronic states, the spectroscopic parameters have been determined in the present work. With the PECs at the present level of theory, vibrational states have been predicted for each state when the rotational quantum number J to zero (J = 0). Not only the question that whether the potential barrier on the ground-state PEC exists or not has been solved, but also the accurate dissociation energy De has been obtained.Secondly, the PECs of electronic states (X~1Σ+, a~3Π, A~1Π, b~3Σ+, B~1Σ+)of BeO molecule have been investigated using the CASSCF method followed by the MRCI approach, with the cc-pV5Z basis set for Be and aug-cc-pV6Z basis set for O. The spectroscopic parameters are determined for the five states, using the PECs of BeO molecule, with the help of VIBROT module in MOLCAS 7.4 program package. With the PEC of the electronic state X~1Σ+, vibrational states have been predicted for each state when the rotational quantum number J to zero (J = 0). Comparison with the available experiments and theories shows that the present calculations are both reliable and accurate.Finally, the ground PEC of Al16O molecule has been calculated using the CASSCF method followed by the MRCI approach, with the basis set aug-cc-pV6Z. The spectroscopic parameters and vibrational manifolds are determined for the ground states of Al16O and Al18O, using the PEC of Al16O molecule, with the help of VIBROT module in MOLCAS 7.4 program package.