Mineral medicine,especially those containing heavy metals,is one of the characteristics of traditional Chinese medicine.A famous mineral medicine,realgar,containing heavy metal arsenic with a chemical formula of As4S4,has the function of detoxification,killing bacteria and viruses,and eliminating dampness and phlegm.Different As4S4 isomers are likely to have different drug effects and pharmacological actions.Therefore,it is of great scientific significance to find more stable AS4S4 isomers.In view of this,ab initio molecular orbital theory and density functional theory (DFT) have been used to study ten isomers of As4S4 at the B3LYP/6-31G*,B3LYP/6-311+G*,B3LYP/6-31 l+G(3df,2p) and MP2/(6-311+G*,LanL2MB) levels of theory.In addition to the two isomers having been studied previously,eight new isomers were investigated in the present paper.All the ten As4S4 isomers were proved to be true local minima on their potential energy surfaces.The calculated NICS values and molecular orbital analyses showed that,the D2d symmetric As4S4,isomer 1,may be σ-aromatic.The study proves that ten As4S4 isomers are stable thermodynamically,and are highly desirable for the future theoretical study of realgar.