Quantum Chemical Calculation Study Of SF6 Molecular Vibration

SF₆ gas is a commonly used insulating gas in industrial electrical equipment and a greenhouse gas.It is very important to detect its content.Infrared and Raman spectroscopy are effective detection methods for SF₆.Both infrared and Raman spectroscopy are related to molecular vibrations.In addition,the influence of the vibration of SF₆ molecules on the higher harmonics generations is also an important topic in the physics of strong laser fields.In this paper,the following work is done on the calculation of the vibration frequency of SF₆,the symmetry of the highest occupied orbital of SF₆,and the impact of molecular vibration on higher harmonic generations.Firstly,this paper proposes a method to save computational cost for the quantum chemical calculation of normal vibrational frequencies of larger molecules.In principle,the quantum chemical calculation of the vibration frequency requires the calculation of the Hessian matrix of the 3N-dimensional potential energy hypersurface and its second derivative.In this paper,it is proposed that the approximate expression of the potential energy curve in a single vibration mode can be derived based on the simple harmonic oscillator,Then,the potential energy curve corresponding to a single vibration mode is calculated by quantum chemical method,and the potential energy curve is fitted into the expression to obtain the vibration frequency.So,we convert 3N-dimensional quantization calculations into one-dimensional calculations.The saved computational resources allow us to choose higher-level computational methods and use more complete basis sets to improve computational accuracy.In this paper,we first take the calculation of the vibrational frequency of the B₂ vibrational mode of water molecules as an example to illustrate the feasibility of this method.Then we apply this method to SF₆ molecules.The MRCI method is an effective method to calculate the electron correlation energy.In this paper,the MRCI/6-311G*basis set is used to calculate the vibration frequencies of the four vibration modes of SF₆,namely A_1g,E_g,T_2g and T_2u.In comparison,the relative errors of the four frequencies calculated in this paper are the smallest.Another work in this paper is to determine the symmetry of the highest occupied orbital of the SF₆ molecule.The symmetry of the highest occupied orbital of SF₆ calculated by ab initio method is directly described by the symmetry of the D_2h group,which needs to be corresponding to the symmetry of the O_h group to which SF₆ belongs.We determined that the symmetry of highest occupied orbital of SF₆ calculated by the Hartree-Fock method is T_1g using the signature projection operator.Finally,the impact of vibration on higher harmonic generations is a hot spot in the field of strong fields.This paper reviews the effects of molecular vibrations on the frequency and amplitude of higher harmonic generations,and summarizes the progress of applying higher harmonic generations to study molecular vibrational dynamics.Prepare for further study of the effect of vibration on higher harmonics in SF₆ molecules.