  Hydrogen bond patterns, proton ordering, and phase transitions of monolayer
ice in two-dimensional hydrophobic confinement are fundamentally different from
those found for bulk ice. To investigate the behavior of quasi-2D ice, we
perform molecular dynamics simulations of water confined between fixed graphene
plates at a distance of 0.65 nm. While experimental results are still limited
and theoretical investigations are often based on a single force field model,
this work presents a systematic study using different water force fields, i.e.
SPCE, TIP3P, TIP4P, TIP4P/ICE, TIP5P. The water occupancy of the graphene
capillary at a pressure of 1000 MPa is determined to be between 13.5 and 13.9
water molecules per square nanometer, depending on the choice of the water
force field. To ensure complete sampling of the configurational space and to
overcome barriers separating metastable structures, these simulations are based
on the replica exchange molecular dynamics technique. We report different
tetragonal hydrogen bond patterns which are classified as nearly square or as
rhombic. While many of these arrangements are flat, in some cases puckered
arrangements are found, too. Also the proton ordering of the quasi-2D water
structures is considered, allowing to identify them as ferroelectric,
ferrielectric or antiferroelectric. For temperatures between 200 K and 400 K we
find several second-order phase transitions from one ice structure to another,
changing in many cases both the arrangements of the oxygen atoms and the proton
ordering. For temperatures between 400 K and 600 K there are melting-like
transitions from a monolayer of ice to a monolayer of liquid water. These
first-order phase transitions have a latent heat between 3.4 and 4.0 kJ/mol.
