Stability-Ranking of Crystalline Ice Polymorphs Using Density-Functional Theory

In this work, we consider low-enthalpy polymorphs of ice, predicted previously using a modified basin-hopping algorithm for crystal-structure prediction with the TIP4P empirical potential at three pressures (0, 4 and 8 kbar). We compare and (re)-rank the reported ice polymorphs in order of energetic stability, using high-level quantum-chemical calculations, primarily in the guise of sophisticated Density-Functional Theory (DFT) approaches. In the absence of applied pressure, ice Ih is predicted to be energetically more stable than ice Ic, and TIP4P-predicted results and ranking compare well with the results obtained from DFT calculations. However, perhaps not unexpectedly, the deviation between TIP4P- and DFT-calculated results increases with applied external pressure.