  We employ the Feedback In Realistic Environments (FIRE-2) physics model to
study how the properties of giant molecular clouds (GMCs) evolve during galaxy
mergers. We conduct a pixel-by-pixel analysis of molecular gas properties in
both the simulated control galaxies and galaxy major mergers. The simulated
GMC-pixels in the control galaxies follow a similar trend in a diagram of
velocity dispersion ($\sigma_v$) versus gas surface density
($\Sigma_{\mathrm{mol}}$) to the one observed in local spiral galaxies in the
Physics at High Angular resolution in Nearby GalaxieS (PHANGS) survey. For
GMC-pixels in simulated mergers, we see a significant increase of factor of 5 -
10 in both $\Sigma_{\mathrm{mol}}$ and $\sigma_v$, which puts these pixels
above the trend of PHANGS galaxies in the $\sigma_v$ vs $\Sigma_{\mathrm{mol}}$
diagram. This deviation may indicate that GMCs in the simulated mergers are
much less gravitationally bound compared with simulated control galaxies with
virial parameter ($\alpha_{\mathrm{vir}}$) reaching 10 - 100. Furthermore, we
find that the increase in $\alpha_{\mathrm{vir}}$ happens at the same time as
the increase in global star formation rate (SFR), which suggests stellar
feedback is responsible for dispersing the gas. We also find that the gas
depletion time is significantly lower for high $\alpha_{\mathrm{vir}}$ GMCs
during a starburst event. This is in contrast to the simple physical picture
that low $\alpha_{\mathrm{vir}}$ GMCs are easier to collapse and form stars on
shorter depletion times. This might suggest that some other physical mechanisms
besides self-gravity are helping the GMCs in starbursting mergers collapse and
form stars.
