Abstract
There are a large number of materials with mild stiffness, which are not as soft as tissues and not as strong as metals. These semihard materials include energetic materials, molecular crystals, layered materials, and van der Waals crystals. The integrity and mechanical stability are mainly determined by the interactions between instantaneously induced dipoles, the so called London dispersion force or van der Waals force. It is challenging to accurately model the structural and mechanical properties of these semihard materials in the frame of density functional theory where the non-local correlation functionals are not well known. Here, we propose a van der Waals density functional named vdW-DFq to accurately model the density and geometry of semihard materials. Using β -cyclotetramethylene tetranitramine as a prototype, we adjust the enhancement factor of the exchange energy functional with generalized gradient approximations. We find this method to be simple and robust over a wide tuning range when calibrating the functional on-demand with experimental data. With a calibrated value q = 1.05 , the proposed vdW-DFq method shows good performance in predicting the geometries of 11 common energetic material molecular crystals and three typical layered van der Waals crystals. This success could be attributed to the similar electronic charge density gradients, suggesting a wide use in modeling semihard materials. This method could be useful in developing non-empirical density functional theories for semihard and soft materials.
Highlights
Energetic materials are a common class of semihard materials with light composites of elements C, H, O, and N, of which the molecular sizes are moderate leading to fast chemical kinetics so that the chemical reactions during decomposition or detonation occurs in a few picoseconds
To gain high energy densities, these molecules are mostly bonded via intermolecular London dispersion forces to form molecular crystals (Figure 1a–k), which belong to semihard materials since they have mild hardness
We assess the performance of van der Waals density functionals in predicting the geometry of semihard materials
Summary
Energetic materials are a common class of semihard materials with light composites of elements C, H, O, and N, of which the molecular sizes are moderate (ranging from 10 to 1000 atoms) leading to fast chemical kinetics so that the chemical reactions during decomposition or detonation occurs in a few picoseconds. To gain high energy densities, these molecules are mostly bonded via intermolecular London dispersion forces to form molecular crystals (Figure 1a–k), which belong to semihard materials since they have mild hardness. The mass density is closely related to the energy density and detonation performance. The accurate prediction of the mass density within 3% of experimental values is critical in predicting the reaction rate and energy density. Density Functional Theory (DFT) [1] is a reliable and popular tool to predict material properties and material designs. The London dispersion force is poorly described with conventional semi-local approximations to DFT [2] since it is essentially
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