The Quantum Chemical Calculation Of SF₆ And S₂

As a good insulating material,an important greenhouse gas and a hyper-valent molecule with the high octahedral_hOsymmetry,the sulfur hexafluorideSF₆ has attracted wide attention for both the fundamental scientific interest and practical industrial applications.For the molecules belonging to high symmetry group,the quantum chemical calculation always adopts the subgroup.Thus the symmetries of some electric states or some molecular orbitals,which belong to different types of representations in high symmetry group,may coincide in the sub-group presentations.Therefore,they cannot be distinguished directly from the sub-group results.So at the first,this paper provides a method to identify the symmetry of molecular orbitals from the theoretical sub-group results and applied this method todetermine the symmetry of the highest occupied molecular orbitals（HOMO）of the sulfur hexafluorideSF₆moleculeas an example.we calculate the orbital functions in the equilibrium structure of ground state ofSF₆ by using HF/6-311G*method,which is implemented with Molpro software.The expressions of the HOMO functions which are triplet degenerate in energy are obtained.Then by acting the symmetric operation ofO_h group on three HOMO functions,we obtain their matrix representations and thus their characters.Finally,the symmetry of the HOMOs is verified to beT_1g.The second part of this paper is to calculate the molecular potential energy curve and vibration frequency by configuration interaction method.Because of the large number of valence electrons ofSF₆,the large amount of calculation,and the symmetry of the normal mode is too complex.At present,most ab initio calculations of potential energy curve and normal mode ofSF₆ only use the HF method,which does not consider the configuration interaction,thus limiting the accuracy of the calculation.In this paper,by considering the normal mode seperately,we calculate the potential curve of the ground state and vibration frequency by using fully active self-consistent field method（CASSCF）and multiple reference configuration interaction method（MRCI）,in which we select proper basis,as well as proper activity space.The equilibrium internuclear distance and vibration frequency by MRCI method has greatly improved compared with HF method.The last part of this paper is to study the predissociation ofS₂molecules.First,the molecular potential energy curve is calculated by using CASSCF/ccp-VQZ method.Then we calculate the Spin-orbital coupling in the cross region of the potential energy curve.By analyzing the fine structure of molecular state,this paper explains the diffusion ofB³（50）_u^-?X³（50）_g^-（v’0,）transition spectrum line for v’?18which observed by Wheeler.