Elastic Constant Calculations for Molecular Organic Crystals

Abstract

Elastic constants of a set of molecular organic crystals have been calculated within the crystal modeling program DMAREL, which was developed to allow the use of highly accurate, anisotropic atom-atom potentials. A set of six molecular crystals (durene, m-dinitrobenzene, the form of resorcinol, pentaerythritol, urea, and hexamethylenetetramine) has been chosen to sample a range of intermolecular interactions and crystal symmetries. The sensitivity of such calculations to variations in empirical repulsion-dispersion parameters and the electrostatic model is determined and discussed relative to the other errors in the theoretical model and typical experimental uncertainties. We find that model potentials whose functional form has been simplified often reproduce crystal structures and lattice energies adequately but perform poorly when used to calculate elastic constants. This is because more theoretically justified potentials are required to satisfactorily model the curvature at the equilibrium geometries. The rigid-molecule approximation can result in exaggerated elastic constants, and the neglect of thermal effects also leads to significant overestimation of the stiffness constants.

Probing the Anisotropic Field‐Effect Mobility of Solution‐Deposited Dicyclohexyl‐α‐quaterthiophene Single Crystals

“…This rigid-molecule approximation drastically accelerates the convergence of the energy optimization but yields reasonably accurate results only if the molecule is sufficiently stiff. Small molecules that possess an extended delocalized p-electron system (like CH4T) are often modeled as being rigid [26,27], neglecting possible twist between the aromatic units in the solid phase (compared to the geometry of the isolated molecule). It is noted that while the force field parameters and equilibrium bond lengths are usually derived for room temperature, effects like anisotropic thermal expansion of the crystal by vibronic excitations are not captured at the level of theory that molecular force fields are based on.…”

Thermal properties of molecular crystals through dispersion-corrected quasi-harmonic ab initio calculations: the case of urea

“…Only in recent years, have different theoretical approaches been devised in order to predict their structural and energetic properties (with the main goal of discriminating between competing polymorphs): from force-field to high-level molecular fragment-based schemes, from periodic dispersion-corrected density functional theory (DFT-D) to periodic many-body wave-function techniques. [10][11][12][13][14][15][16][17][18] However, once a reliable and balanced description of the various chemical interactions has been achieved by means of any of the above-mentioned quantum-chemical methods, the extension of their applicability to more complex properties of technological and industrial relevance, which would greatly increase their predictiveness, such as mechanical, elastic, optical and thermodynamic responses, [19][20][21][22] has to be performed. Apart from the intrinsic high degree of complexity of the required theoretical techniques and algorithms, the main difficulty here is represented by the fact that most of those properties are largely affected by thermal effects, [23][24][25] even at room temperature, such as zero-point energy, harmonic and anharmonic thermal nuclear motion, thermal lattice expansion, etc.…”

Improving Mechanical Properties of Crystalline Solids by Cocrystal Formation: New Compressible Forms of Paracetamol

“…To further explore the relationships between crystal packing and mechanical properties, we have estimated the elastic constants of each cocrystal from the second derivatives of their calculated lattice energies, using the program DMAREL. [24] Such calculations have previously been used to rationalize the mechanical property differences between pca forms 1 and 2.…”