Water has been studied extensively by computer simulation
using both the molecular dynamic and Monte Carlo
methods, and consequently valuable insight has been gained
about the nature of the liquid and solid
phases. In the
various studies, many different potentials have been used to
model the molecular interactions. These range from simple
rigid molecule interactions such as the ST2 potential to
more complex potentials which allow for molecular vibrations
and chemical reactivity. Considering the obvious importance
of phase transitions, it is indeed surprising that
only recently has the phase behavior of water been examined
via molecular dynamics simulation. I I A necessary but not
sufficient test of any potential used to model water is that the
density should increase as melting occurs. A corollary to
this, of course, is that applying pressure to the crystal should
cause its melting point to decrease. In this paper, we extend
our previous studies on ice crystal melting at low pressure to
report here the melting of an ice crystal under substantial
applied pressure. In addition, we continue in our effort to
understand the nature and role of amorphous structures in
governing the mechanically stable packing of water molecules
by performing non annealing quenches from various
thermodynamic states generated in the melting studies.
The computer simulations (detailed in Sec. II) are performed
on small clusters of 250 molecule ST2 water molecules constrained
by an external potential. These studies differ from
the procedure normally followed in molecular dynamic
simulations in that no periodic boundary conditions are applied
and also in that no cutoff on the force are used. It is
essential that the clusters have a free surface because ice
melting is normally initiated at the surface and proceeds inward
to the center of the crystal. In our previous simulation
of ice melting ll we had applied a weak external potential
which prevented the droplet from evaporating as melting
occurred and which acted as a nonwetting container for the
cluster. We now use this same potential function with suitably
modified parameters to compress the droplet. Although
the requirement of no cutoff on the force makes the calculations
more difficult, this is necessary to maintain the validity
of the calculations, particularly since the droplet has a free
surface.
In Sec. III, we discuss the thermodynamic properties of
the cluster as a function of temperature. We find as in our
previous simulations that the crystal starts to melt at the
surface and that the melting proceeds inward to the core
region until the entire ice crystal structure is consumed. We
also find that the pressure produces a depression of the melting
point.
In Sec. IV, we detail the results of some very rapid
quenches of various thermodynamic states produced in following
the crystal melting and fluid heating curve. About
80%-90% of the latent heat of melting is attributable to the
upward shift in the potential energy as the underlying inherent
structures shift from regular packing to amorphous
packing.
This paper ends with a discussion of the results and the
remaining important issues.