Energy landscapes of the water hexamer and octamer for the MB-pol and TIP4P/2005 potentials

Among the numerous interaction potentials available for water, MB-pol and TIP4P/2005 are popular choices to model both gas and condensed phases. Here, we examine the intermediate case of finite clusters for 6 and 8 molecules, with a thorough survey of the energy landscapes using a combination of basin-hopping global optimization and discrete path sampling methods. The two potentials predict qualitatively similar energy landscapes for both the hexamer and octamer. However, important differences arise due to changes in the energetic ordering of corresponding minima, including the global minimum itself. Differences in the classical heat capacities are relatively modest and mostly occur at low temperatures as a result of transitions between competing isomers. The heat capacity peaks can be assigned and interpreted using the temperature gradient of the occupation probabilities and involve only a few minima. The polarizable and flexible MB-pol model is more sophisticated than the rigid, non-polarizable TIP4P/2005 model and produces different intermediates in the isomerization pathways between low-energy configurations of the hexamer. In contrast, the pathways connecting low-lying cubic isomers of the octamer agree quantitatively between the two models, aside from some energetic reordering.