CO分子四个电子态的振转谱:两种效应修正方法的比较

基于完全活性空间自洽场方法和多参考组态相互作用（multi-reference configuration interaction method，MRCI）方法，采用MRCI+Q/CBS（TQ5）+CV+SR（方法A）和aug-cc-pwCVnZ-DK（n=T，Q，5）（方法B）方案，分别计算了包含Davidson修正（+Q）、芯-价电子关联（core-valence correlation correction，CV）效应以及标量相对论（scalar relativistic，SR）效应的CO分子的基态X¹∑⁺和激发态a³Π，a'³∑⁺和A¹Π的势能曲线.在此基础上，获得了这些电子态的振-转谱.通过与实验结果比较发现：方法A适合a'³∑⁺和A¹Π等较高激发态的振-转谱的计算，方法B更适合基态X¹∑⁺和第一激发态a³Π的振-转谱的精细计算.该研究可以为其他小分子高精度振-转谱快速计算方案选择提供参考.

Rovibrational spectrum calculations of four electronic states in carbon monoxide molecule: Comparison of two effect correction methods

Accurate calculation of molecular energy is of great significance for studying molecular spectral properties. In this work, the potential energy curve and rovibrational spectrum (G_ν) of the ground state X¹∑⁺ and the excited states a³Π, a'³∑⁺ and A¹Π of carbon monoxide molecule are calculated by the multi-reference configuration interaction method. In the calculation, the core-valence correlation correction (CV) effect and scalar relativistic (SR) effect are included.
In order to obtain an accurate energy of molecule, two computational schemes are adopted. In the first scheme, i.e. (m MRCI+Q/CBS(TQ5)+CV+SR), the molecular orbital wavefunction is obtained from the Hartree-Fock self-consistent field method by using the basis set aug-cc-pVnZ. The wavefunction is first calculated by the state-averaged complete active space self-consistent field approach. Then the multi-reference configuration interaction method (MRCI) is adopted to calculate the dynamic correlation energy in the potential energy curve. Finally, we use the basis set cc-pCVQZ and aug-cc-pVQZ to calculate the CV effect and SR effect by the MRCI method. In the second scheme (aug-cc-pwCVnZ-DK (n=T, Q, 5)), the potential energy curves (PECs) of these four electronic states are calculated by the MRCI method whose basis set (aug-cc-pwCVnZ-DK) contains the CV effect and SR effect. Finally, in order to reduce the error caused by the basis set, we extrapolate the basis sets of the two computational schemes to the complete basis set. On the basis of the PECs plotted by the different methods, we obtain the spectroscopic parameters of the X¹∑⁺, a³Π, a'³∑⁺ and A¹Π states of the carbon monoxide by solving the internuclear Schrödinger equations through utilizing the numerical integration program “LEVEL”.
In this paper, we calculate the SR effect and the CV effect by using different schemes, and the latter is indispensable for accurately calculating the molecular structure. For the lowest two electronic states, we consider the dependence of the two effects on the calculation of the Gaussian basis group (Method B), and find that the accuracy of the rovibrational spectrum is improved. It can be seen that these electronic states have higher requirements for electronic correlation calculation. For higher electronic states, the electron cloud distribution is relatively loose, and the electronic correlation obtained by a single Gaussian basis group can achieve the corresponding calculation accuracy. Of course, since the calculation of the rovibrational spectra is essentially only the relative energy, the offset effect of the electronic correlation effect of different electronic states is also included here in this paper.