Configurational Entropy in Ice Nanosystems: Tools for Structure Generation and Screening

Recently, a number of experimental and theoretical studies of low-temperature ice and water in nanoscale systems have emerged. Any theoretical study trying to model such systems will encounter the proton-disorder problem, i.e., there exist many configurations differing by water-molecule rotations for a fixed oxygen atom structure. An extensive search within the allowed proton-disorder space should always be perfomed to ensure a reasonable low-energy isomer and to address the effect of proton-configurational entropy that may affect experimental observables. In the present work, an efficient general-purpose program for finite, semiperiodic, and periodic systems of hydrogen-bonded molecules is presented, which can be used in searching and enumerating the proton-configurational ensemble. Benchmarking tests are performed for ice nanotubes and finite slabs. Finally, the program is applied to experimentally appropriate ice nanosystems. A boron nitride film supported ice nanodot is studied in detail. Using a systematic generation of its proton-configurational ensemble, we find an isomer that is ∼1 eV lower in total energy than one previously studied. The present isomer features a considerable dipole moment and implies that ice nanodots are inherently ferroelectric parallel to the surface. We conclude by demonstrating how the so-called hydrogen-bond connectivity parameters can be used to screen low-energy isomers.