Structure and stability of XeF₆ isomers: Density functional theory study and the maximum hardness principle

The geometrical configurations of the four possible isomers with C_3v, O_h, C_s and C_2v symmetry on the potential energy surface of the XeF₆ molecule are optimized by using DFT-LDA/NL. Their relative energies, vibration frequencies, electronic chemical potential and hardness have been calculated. It is found that the C_2v configuration has one imaginary frequency. The relative energies of the four isomers increase in order of C_3v, O_h, C_s and C_2v, and the hardness values in same order. The isomer stability obeys the maximum hardness principle (MHP), while their hardness values are very close to each other. It is quite evident that the very close hardness is the main reason for the structure fluxionality of XeF₆.