  Photo-induced insulator-metal transition in VO$_2$ and the related transient
and multi-timescale structural dynamics upon photoexcitation are explained
within a unified framework. Holes created by photoexcitation weaken the V-V
bonds and eventually break V-V dimers in the M$_1$ phase of VO$_2$ when the
laser fluence reaches a critical value. The breaking of the V-V bonds in turn
leads to an immediate electronic phase transition from an insulating to a
metallic state while the crystal lattice remains monoclinic in shape. The
coupling between excited electrons and the 6.0 THz phonon mode is found to be
responsible for the observed zig-zag motion of V atoms upon photoexcitation and
is consistent with coherent phonon experiments.
