Browsing by Author "Shahbaz, Muhammad"
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Item Crystal Structure Predictions for 4-Amino-2,3,6-trinitrophenol Using a Tailor-Made First-Principles-Based Force Field(Crystal Growth and Design, 2022-01-24) Metz, Michael P.; Shahbaz, Muhammad; Song, Hongxing; Vogt-Maranto, Leslie; Tuckerman, Mark E.; Szalewicz, KrzysztofPredictions of crystal structures from first-principles electronic structure calculations and molecular simulations have been performed for an energetic molecule, 4-amino-2,3,6-trinitrophenol. This physics-based approach consists of a series of steps. First, a tailor-made two-body potential energy surface (PES) was constructed with recently developed software, autoPES, using symmetry-adapted perturbation theory based on a density-functional theory description of monomers [SAPT(DFT)]. The fitting procedure ensures asymptotic correctness of the PES by employing a rigorous asymptotic multipole expansion, which seamlessly integrates with SAPT(DFT) interaction energies. Next, crystal structure prediction (CSP) was performed by generating possible crystal structures with rigid molecules, minimizing these structures using the SAPT(DFT) force field, and running isothermal–isobaric molecular dynamics (MD) simulations with flexible molecules based on the tailor-made SAPT(DFT) intermolecular force field and a generic/SAPT(DFT) intramolecular one. This workflow led to the experimentally observed structure being identified as one of the forms with the lowest lattice energy, demonstrating the success of a first-principles, bottom-up approach to CSP. Importantly, we argue that the accuracy of the intermolecular potential, here the SAPT(DFT)-based potential, is determinative of the crystal structure, while generic/SAPT(DFT) force fields can be used to represent the intramolecular potential. This force field approach simplifies the CSP workflow, without significantly compromising the accuracy of the prediction.Item Dispersion energy in density-functional theory(University of Delaware, 2019) Shahbaz, MuhammadDensity Functional Theory (DFT), in various local and semilocal approximations, cannot completely describe long-range correlations between the electrons responsible for dispersion interactions. A large number of methods have been designed to correct DFT for the missing dispersion effects (DFT+D methods). These methods add a fraction of true dispersion energy to DFT methods assuming that a part of it has already been recovered by DFT. We estimate the amount of dispersion recovered by different popular DFT methods and show that what appears to be recovered dispersion energy does not possess the physical character expected of dispersion interactions. Moreover, a large part of it originates from those terms of the DFT interaction energy that do not have any physical mechanism to capture such effects. The technique used to estimate the recovered dispersion will help for future developments of DFT methods as it points out the shortcomings of the dispersionless parts of the DFT interaction energy as well. A new method for calculating dispersion interactions is also developed using a modified polarizability density from nonlocal correlation methods. The performance of the new method is tested on a set of dimers at various intermonomer separations. The new method outperforms all nonlocal correlation functionals and reduces the average error on the test set by at least a factor of 2. Finally, a path for the future development of nonlocal correlation methods is provided by comparing polarizability densities from nonlocal correlation functionals to the accurate one provided by time-dependent DFT.