Computational Studies of Crystal Structure and Bonding - Advanced X-Ray Crystallography - page 28

Fig. 12 Symmetrization run for n -hexane. Left : the evolution of the overall asymmetry index
( upper curve ) and of the overall density ( lower curve ). Right : the evolution of pressure (atm) and
of intramolecular, intermolecular Lennard-Jones and total energy (kJ mol 1 ). The abscissa shows
the number of million MC moves, corresponding to a time of a few picoseconds
This forced symmetrization procedure has been applied [ 62 ] to simulate the
transition of an n -hexane computational box from isotropic liquid to a semicrystal-
line phase. The result is that molecules become aligned as they are aligned in the
real crystal structure, although the final, complete crystalline periodic symmetry
cannot be reached; the time barrier is clearly still operating. The interesting result
is not, however, the achieved linearization (this is just a confirmation that the
algorithm has been properly programmed into the computer). The novel insight is
contained in the evolution trajectory, depicted in Fig. 12 , which shows that, as the
asymmetry index decreases and molecules are brought into a parallel configuration,
internal pressure goes up, and the system must overcome a barrier in which the
internal energy rises and the density decreases. The result does not change on
change of force fields, although detail of the tolerance imposed on the observance
of the bias influences the height of the barriers. The simulation suggests that incre-
asing symmetrization induces localized micro-expansions as molecules rearrange
and find their way to the new configuration; for n -hexane, this process may involve
a request of extra space as molecules uncoil from the partially gauche conforma-
tions in the liquid to the all- trans conformation in the solid.
6.3 What Is the Real Value of a Simulation?
The scope of evolutionary simulation is presently largely limited by the size and time
problems. Its force fields are mostly expert systems and are far from a faithful
reproduction of real physical effects - think of the wrong description of short-
range Coulombic forces resulting from locating formal charges at atomic nuclei,