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The common form of ice crystals is known as hexagonal ice. In this
form the oxygen atoms of the water molecules are arranged in a
tetrahedral lattice. Each water molecule is bound to four neighboring
molecules by means of bridging hydrogen bonds, leading to an average
of two bridges per molecule. In water, there are, on average, only
1.75 bridging hydrogen bonds per molecule.
What happens in the process of melting" Carl Caleman and David van der
Spoel have now successfully used a computer to simulate �snapshots� of
melting ice crystals. These molecular dynamics simulations are ideal
for gaining a better understanding of processes like melting or
freezing because they make it possible to simultaneously describe both
the structure and the dynamics of a system with atomic resolution and
with a time resolution in the femtosecond (10-15 s) range.
The simulation demonstrated that the energy of the laser pulse
initially causes the OH bonds in the water molecules to vibrate.
Immediately after the pulse, the vibrational energy reaches a maximum.
After about a picosecond, most of the vibrational energy has been
transformed into rotational energy. The molecules begin to spin out of
their positions within the crystal, breaking the bridging hydrogen
bonds. After about 3 to 6 picoseconds, the rotations diminish in favor
of translational motion. The molecules are now able to move freely and
the crystal structure collapses. This process starts out locally, at
individual locations within the crystal. Once the symmetry of the
structure is broken, the likelihood of melting processes occurring in
the area immediately surrounding the crystal defect rises
significantly. The melting process thus spreads out from this point
little by little. At other locations the ice can maintain its
crystalline structure a little longer.
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