Periodic DFT Investigation of Isosymmetric Alpha–Beta Phase Transition in Resorcinol Under Ambient and High Pressure

Isosymmetric phase transitions driven by subtle hydrogen-bond rearrangements remain challenging for periodic density functional theory (DFT), particularly when energy differences between polymorphs are small. Resorcinol represents an interesting case in which the α and β polymorphs crystallize in the same space group and differ primarily in hydroxyl orientation and hydrogen-bond topology. In this work, the α–β phase transition was systematically investigated using periodic DFT calculations under ambient and elevated pressure. A broad set of exchange–correlation functionals combined with different dispersion corrections was benchmarked against experimental structural and energetic data. Dispersion-corrected methods were essential for reproducing lattice parameters and the pressure-induced inversion of stability. PBESOL with Tkatchenko–Scheffler dispersion provided the most consistent agreement with the experiment and was therefore used for phonon and ab initio molecular dynamics simulations. Phonon-derived thermodynamic analysis revealed a delicate enthalpy–entropy balance governing the transition, strongly affected by pressure. Dynamical simulations confirmed the instability of the α phase under compression, demonstrating the cooperative nature of this hydrogen-bond-driven isosymmetric transformation.