The Quantum Chemical Calculation Of O₂and CO

Quantum chemical calculation method is one of main tools to study the molecular level structure and spectral characteristics. It begins with the calculation of the potential energy surface and dipole moment surface from which the frequency and the intensity of the spectrum can be obtained. In this work, Quantum chemical software MOLPRO is used to calculate the potential energy curves and dipole moment curves of diatomic molecules O2and CO. In detail:(1) The input file for calculating O2molecule is designed. And the corresponding output file is explained. It is stressed how to use D2h group to represent the electronic states of O2which belongs to D∞h group. It is the basis for the following work.(2) The dipole moment curve of ground state of CO is calculated using the quantum chemical methods with MOLPRO software. The methods used include HF, CASSCF and MRCI. The basis sets used include6-31G,6-31G (d),6-31G (2d,3f) and aug-cc-pv5z.12dipole moment curves dependent on nuclear distance are obtained. The dipole moment values and the dipole moment slopes at equilibrium position are compared with the ones deduced from experimental spectral data. It is found the dipole moment from MRCI/aug-cc-pv5z is best. Its slope value at equilibrium position differs from the experimental ones by less than1%. Then the potential curve of ground state of CO is calculated by MRCI/aug-cc-pv5z. After inserting the potential curve into the Schrodinger equation for nuclear, the vibration energies and force constants are obtained. They are in good agreement with the experimental values.(3) The potential energy curves of the18states of oxygen molecule which dissociated to the same limit O(3P)+O(3P) are calculated using MOLPRO software. The method is CASSCF and the basis set is cc-pvtz. The main electronic configurations of these18states are presented. Then we focus on the three lowest states X3Σg-,b1Σg+and a1Δg. Their potential energy curves are calculated using the methods including CASSCF and MRCI and the basis sets including cc-pvtz, cc-pvqz, and aug-cc-pvqz. By comparing the equilibrium positions and force constant deduced from the potential curves with experimental values, it is concluded that the potential curves from MRCI/avqz are best. Then based on the potential curves form MRCI/avqz, vibration energies and rotational constants are calculated and compared with the experimental values, it is found the relative error is less than1%. Also the potential energy curves of the three states(5Πg、1Πg、3Πg)from MRCI/vqz and MRCI/avqz, which are not observed on experiment, are presented. And the corresponding equilibrium positions and dissociation energies are given.