  Polymorphs in molecular crystals are often very close in energy, yet they may
possess markedly different physical and chemical properties. The understanding
and prediction of polymorphism is of paramount importance for a variety of
applications, including pharmaceuticals, non-linear optics, and hydrogen
storage. Here, we show that the non-additive many-body dispersion (MBD) energy
beyond the standard pairwise approximation is crucial for the correct
qualitative and quantitative description of polymorphism in molecular crystals.
This is rationalized by the sensitive dependence of the MBD energy on the
polymorph geometry and the ensuing dynamic electric fields inside molecular
crystals. We use the glycine crystal as a fundamental and stringent benchmark
case to demonstrate the accuracy of the DFT+MBD method.
