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Low-Cost Strategies for Accurate Free Energy Prediction of Organic Molecular Crystals


Consistent, accurate in silico characterization of organic molecular crystals would greatly benefit the discovery of new materials. This thesis presents several strategies that have been developed for assessment of thermodynamic stability, quantifying finite temperature effects, and prediction of mechanical properties for organic molecular crystals.

Finite temperature contributions to crystal stability are approximated using hybrid semi-empirical and first principles density functional theory for lattice dynamics. This model approximates the phonon density of states using a composite density functional tight binding (DFTB) and density functional theory (DFT). This composite method is applied to oxalyl dihydrazide (ODH) polymorphs and is able to correctly rank the five crystals in relative thermodynamic stability. Absolute values of the thermodynamic state functions are predicted within 1 kJ/mol of their pure DFT values. The combined DFT/DFTB phonon densities of state (pDOS) result in similar accuracy compared to conventional DFT but require far less computational effort.

The thermal expansion of resorcinol, naphthalene, BTBT, and pentacene are also predicted using the DFT/DFTB pDOS along with the quasiharmonic approximation (QHA). The solid state phase diagram for the polymorphic α- and β-resorcinol forms was evaluated, with a predicted transition temperature of 368 K at ambient pressure, in excellent agreement with the literature.

Finally, crystal structure prediction (CSP) was performed for 9-methyl anthracene (9MA), a photomechanical crystal. These crystals undergo a photochemical reaction in the solid state, causing abrupt changes in crystal structure ultimately converting light energy to mechanical work. CSP was used to identify potential polymorphs of the reactant and product 9MA as well as their energetic relationships. The predicted product crystals were verified experimentally. Theoretical work densities for 9MA, 9-tert-butyl anthracene ester, and 9-carboxylic acid anthracene were also predicted to be on the order of ~107 J/m3, making these types of systems incredibly promising for photoactuators.

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