Spectroscopic Properties Investigations Of Several Low-lying Electronic States On Diatomic Molecules Or Cation

The molecular potential energy function is one of the important methods to study the electronic structure of a molecule.It describes the spectroscopic properties of the molecule in the whole space.Therefore,the potential energy function is the foundation of studying molecular reaction dynamics,atomic and molecular collision and other disciplines.In this paper,we calculated the potential energy curve of the ion by using the MRCI method after optimizing the space by the fully active space self-consistent field method(CASSCF).We used the multiple reference method(MR)when considering the influence of the electronic correlation effect in the calculation process.The most commonly used MR method is the multi-reference configuration self-consistent field method(MCSCF)and the internal contraction multi-reference configuration interaction method.The difficulty in the application of MCSCF is the choice of configuration.The CASSCF method is used commonly.The CASSCF method is to select the occupied orbit of molecules and divide the natural orbits into active orbits and inactive orbits.The CASSCF method only considers some of the dynamic correlation energy.In order to obtain accurate results,the MRCI calculation considering the dynamic correlation energy is performed next.However,the MRCI method only considers single and double electron excitations,and there is a"size consistency"error.There are a number of corrections that can solve this problem.This article uses Davidson’s correction to correct the size consistency error produced by MRCI.In this paper,the potential functions,spectroscopic constants and molecular constants of SiP~+and BeC~+ions are mainly studied.X~3Σ_-,A~3Π,1~5Σ~-,d~5Π,2~1Σ~-,c~1Σ~+,b~1Π,2~1Π,3~1Π,a~1Δ,1~1Δ,2~3Σ~-,1~3Σ~+,2~3Π,3~3Π,4~3Π,1~3Φ,1~3Δand 2~3Δstates generated by the first two dissociation channel of SiP~+ion and X~4Σ~-,a~2Π,A~4Π,c~2Σ~-,b~2Δ,d~2Σ~+,e~2Π,2~2Σ~+,3~2Πand 3~2Σ~+states of BeC~+ion are calculated by the internally contracted multi-reference configuration interaction approach.The Davidson correction is taking into account in the calculation.Meanwhile,core-valence correlation and scalar relativistic corrections on the low excited states of diatomic ions are included.All the calculations of ion potential function are carried out in the MOLPRO program to obtain the potential energy curves of the electronic states,and then the obtained potential energy curve is fitted by the MOLCAS program to obtain the spectroscopic constants T_e,D_e,R_e,ω_e,ω_eх_e,ω_ey_e,B_e andα_e.For SiP~+ion and BeC~+ions,the core-valence correlation correction is put into practice with a cc-pCVTZ basis set and the scalar relativistic corrections is considered at the level of a cc-pVTZ basis set,in addition,we extrapolate the potential energy curves(PECs)to the complete basis set limit.In this paper,the transition properties of SiP~+ions and BeC~+ions are studied.The calculation is carried out in the LEVEL program.The Franck-Condon factors and radioactive lifetimes of transitions A~3Π-X~3Σ~-,b~1Π-a~1Δand b~1Π-X~3Σ~-for the X~3Σ~-,A~3Π,a~1Δand b~1Πstates are obtained.