CO2 packing polymorphism under pressure: mechanism and thermodynamics of the I-III polymorphic transition

In this work we describe the thermodynamics and mechanism of CO$_2$ polymorphic transitions under pressure from form I to form III combining standard molecular dynamics, well-tempered metadynamics and committor analysis. We find that the phase transformation takes place through a concerted rearrangement of CO$_2$ molecules, which unfolds via an anisotropic expansion of the CO$_2$ supercell. Furthermore, at high pressures we find that defected form I configurations are thermodynamically more stable with respect to form I without structural defects. Our computational approach shows the capability of simultaneously providing an extensive sampling of the configurational space, estimates of the thermodynamic stability and a suitable description of a complex, collective polymorphic transition mechanism.