  We investigate the vibrational properties of VO2, particularly the low
temperature M1 phase by first-principles calculations using the density
functional theory as well as Raman spectroscopy. We perform the structural
optimization using SCAN meta-GGA functional and obtain the optimized crystal
structures for metallic rutile and insulating M1 phases satisfying all expected
features of the experimentally derived structures. Based on the harmonic
approximation around the optimized structures at zero temperature, we calculate
the phonon properties and compare our results with experiments. We show that
our calculated phonon density of states is in excellent agreement with the
previous neutron scattering experiment. Moreover, we reproduce the phonon
softening in the rutile phase as well as the phonon stiffening in the M1 phase.
By comparing with the Raman experiments, we find that the Raman-active
vibration modes of the M1 phase is strongly correlated with the V-V dimer
distance of the crystal structure. Our combined theoretical and experimental
framework demonstrates that Raman spectroscopy could serve as a reliable way to
detect the subtle change of V-V dimer in the strained VO$_2$.
