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The stable minima of the EX2 molecule(E=C,Si,Ge,Sn,Pb,and X=O,S,Se)with linear and cyclicstructures were calculated at CCSD(T)and B3LYP levels.Most dioxides favor linear structure following Walshrules.However,lead dioxide is stabilized as cyclic structure.For sulfides both SnS2and PbS2are found to belowest in energy while CS2and SiS2keep linear structure as most stable formula.As for selenides more cyclicstructures are located in lower energy than linear structures.The great difference between the linear and cyclicstructures was mainly caused by relativistic effects,where greater size difference between valence s and p atomicorbital existed in heavier atoms have lower tendency to form sp hybrid orbital of linear species.The cyclic structurewas analyzed in detailed according to NBO data and selected molecule orbital finally.