Ab Initio Calculation Of The Potential Curve Of B³?_u^-State Of O₂

B3?u-state of O2 molecule is the upper state of the most strongly allowed triplet-triplet absorption,the Schumann-Runge(SR)transition,which is crucial for protecting the earth from UV radiation.Photo-dissociation of O2 molecule in the SR continuum is the major source of odd oxygen(0 and O3)in the stratosphere.Comprehensive knowledge of the electronic states,especially their potential energy curves,is necessary to understand those phenomena.In this paper,the potential curve of B3?u-state of O2 is investigated by using the internally contracted multi-reference configuration interaction including Davison correction(icMRCI+Q)method.First,we carried out the state average calculation for the lowest five states with the same symmetry as B3?u-state where the multi-reference configurations consist of valence configurations.The obtained potential curve of B3?u-state does not possess double well.It is found that the double well in reference(Spectrochimica Acta Part A:Molecular and Biomolecular Spectroscopy 2014 124 216)comes from the "root flipping".In order to enhance the computing accuracy,by using CASSCF calculate the potential curve of B3?u-state with different active spaces,we discuss the basis set and the reference configuration space influence on potential curve.The augmented correlation consistent ang-cc-pVXZ(AVXZ,X=3,4,5)basis set are used.And we find that the Rydberg configurations with 2?u orbital occupied play a crucial role in forming the bound well of the potential curve.And we find that the spectrum parameters of B3?u-state is convergent respect to the basis set and level of theory.Finally,we add the Rydberg configurations into the multi-reference configurations by adding 2?u orbital into the active space and carry out the calculation at icMRCI+Q level.The obtained B3?u-state potential curve and its molecular constants have more agreements with the experimental data compared with other theoretical calculations.