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.