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Recent experiments have revealed an intriguing pressure-induced isostructural transition of the low temperature monoclinic VO2 and hinted to the existence of a new metallization mechanism in this system.The physics behind this isostructural phase transition and the metallization remains unresolved.In this work,we show that the isostructural transition is a result of pressure-induced instability of a phonon mode that relates to a CaCl2-type of rotation of the oxygen octahedra which alleviates,but does not completely remove,the dimerization and zigzagging arrangement of V atoms in the M1 phase.This phonon mode shows an increasing softening with pressure,ultimately leading to an isostructural phase transition characterized by the degree of the rotation of the oxygen octahedra.