Molecular Dynamics Simulation of Molecular Crystals under Anisotropic Compression: Bulk and Directional Effects in Anthracene and Paracetamol

Parrinello–Rahman
pressure-control algebra with an anisotropic
external stress field, coupled with recently developed, accurate atom–atom
potentials, has been incorporated into new modules of the Milano Chemistry
Molecular Simulation (MiCMoS) computer program package for application
in molecular dynamics (MD) simulations for organic crystals. Simulations
were carried out for two widely different intermolecular environments,
anthracene and paracetamol. The results reproduce quantitatively the
anisotropic evolution obtained by pressure-dependent X-ray diffraction
experiments in hydrostatic conditions. A less usual application concerns
the probing of differently oriented uniaxial stresses, which for anthracene
reveal a phase transition triggered by mechanical excitation along
a direction parallel to the main molecular axis. For paracetamol,
differences in compressibility along different crystal directions
are borne out and are explained in molecular terms with reference
to the hydrogen bonding scheme. Simulations of tensile stress, that
is, negative pressure along different crystal directions, provide
an estimate of yield points in a range of 0.2–0.5 GPa (2000–5000
atm) and indicate the weakest directions. On the basis of these results,
we strongly suggest that classical MD in the atom–atom formulation,
even in the absence of thorough treatment of quantum effects, but
endowed with flexible algebra and coupled with carefully calibrated
intermolecular potentials, can give reliable results of quantitative
and semiquantitative character on the structural dynamics of organic
crystals, providing an essential support to downstream studies of
mechanical, optical, and electronic properties. Widespread use for
further experience and validation is encouraged by the availability
of the Fortran source codes.