Theoretical Investigation On The Irregular Potential Energy Functions Of Diatomic Molecules

Potential energy function is the complete description of electronic structure for molecule itself. Not only the molecular energy, geometrical configuration, force constant, spectroscopic constant are obtained from potential energy function, but it also provides a lot of information for the investigation in the high and new-technology field. For instance, spaceflight, laser, material, and so on. Therefore, it is necessary to accurately investigate the molecular potential energy function from theoretical level. The potential energy function of the diatomic molecule is considered as the basic issues. Because of, it is only the function of the internuclear distance. To date, the major researches in this field mainly are concentrated in the ground state. For the excited state, especially the high-excited state potential energy function, the researches are few.Based on a comprehensive and systematic introduction of molecular potential energy function and correlative quantum chemical methods and a lot of documents, Complete Active Space Self-Consistent Field (CASSCF) method is explored. Then, the investigations on the irregular potential energy curves of the diatomic molecule are calculated by means of CASSCF method. Firstly, with the CASSCF method, provided by the Gaussian03 program package, the potential energy curves for the electronic states X~1∑~+and B~1∑~+of BH molecule are calculated respectively. Not only the analytical form of Murrell-Sorbie function is obtained, but the spectroscopic constant and force constant are also calculated depending on this analytical function. In order to prove the accuracy of the computational results and the reliability of the CASSCF method, Symmetry Adapted Cluster/Symmetry Adapted Cluster Configuration Interaction (SAC/SAC-CI) is used to calculate the two electronic states mentioned above. The results show that the CASSCF result is more accurate than the SAC/SAC-CI, which is considered as a better method. Simultaneously, the deviation between the CASSCF result and experimental data of the B 1∑+electronic state potential energy curve is analyzed rigorously. It shows the shape of the potential curve is affected byω_eχ_e andα_e intensely. So, we believe this mainly dues to the fact that the experimental spectroscopic dataω_eχ_e andα_e are too similar to the theoretical result. Secondly, the potential energy curve of A~2∑~+electronic state for OH molecule is calculated with CASSCF method. Then, fit the computational result into the Murrell-Sorbie function. The spectroscopic data of this electronic state is calculated furtherly. This result shows that, for the open shell molecule, the better result can also be gotten with the CASSCF method. Thirdly, the ground state potential energy curve of the CuIn molecule, which is very controversial, is respectively calculated via the CASSCF method, MP2 method, DFT method. The results show that MP2 result is different from the CASSCF and DFT. The CASSCF result and the DFT result suggest that the potential energy curve of the ground state for the CuIn molecule is a regular curve. However, from the MP2 result, we can see that there exists a maximum on the potential energy curve before dissociation limit. We also give some analyses in this paper.