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

In this work, we describe the thermodynamics and mechanism of CO₂ 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 CO₂ molecules, which unfolds via an anisotropic expansion of the CO₂ 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.