**Increase in local crystalline order across the limit of stability leads
to cubic-hexagonal stacking in supercooled monatomic (mW) water**

N Pingua and PA Apte, JOURNAL OF CHEMICAL PHYSICS, 149, 074506 (2018).

DOI: 10.1063/1.5047464

At the limit of stability of a supercooled tetrahedral liquid modeled by
monatomic (mW) water potential, it was recently shown that relaxation
occurs across a unique value of per particle potential energy
(phi(mid)), which corresponds to a dynamical (non-stationary) condition
of Gibbs free energy function G(T, P, N, phi): **partial derivative(2)
(G/N)/partial derivative phi(2) = 0** and **partial
derivative(G/N)/partial derivative phi not equal 0**. In this work, we
explore the inherent structures responsible for the formation of the
amorphous states through such a mechanism of relaxation of mW liquid. We
first identify 6-member boat and chair shaped rings using a criterion
based on the internal dihedral angles. We then consider the stacking of
the cubic diamond (10-atom cluster with 4 chair shaped rings) and
hexagonal wurtzite (12-atom cluster with 3 boat and 2 chair shaped
rings) units through a shared chair ring. We find that the local
crystalline (tetrahedral) order is exhibited by the eclipsed bond
particles of the laterally connected wurtzite units which are stacked
from both sides with the diamond units (DWD stacking). Increasingly
longer range crystalline order is obtained as the number of stacked
wurtzite layers increases: the particles shared by the stacked
(laterally connected) wurtzite layers in DWWD show a longer range
crystalline order. An even longer range crystalline order is exhibited
by the eclipsed bond particles of the middle (laterally connected)
wurtzite layer of DWWWD stacking. We find that cubic-hexagonal stacking
occurs primarily in the form of DWD layers across the limit of
stability. The local tetrahedral order of the purely cubic (diamond)
network particles (which are not shared with wurtzite units) deviates
significantly from that of the hexagonal crystal. Nonetheless, the
average length of the bonds in the purely cubic network approaches that
in the hexagonal crystal very closely. Thus a large increase in the
purely cubic ice across the instability also leads to an increase in the
local crystalline order in the form of bondlengths. Our results are
consistent with previous experimental and simulation studies which find
a significant fraction of cubic ice along with cubic-hexagonal stacking
layers in deeply supercooled water. Published by AIP Publishing.

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