A van der Waals Density Functional for Molecular Crystals

Since the development of the first van der Waals density functional by Dion et. al., the modeling of non-local correlation has evolved to more accurately describe larger and more varied types of structures. The newest generation of the vdW-DF family, i.e. vdW-DF3, constructed new forms of the non-local correlation functional and exchange and achieved improved accuracy over past van der Waals density functionals in modeling molecular dimers, layered structures, and adsorption systems. However due to competing interests within the parametrization of the functional, only limited accuracy was achieved for molecular crystals. Here we offer a new, highly accurate molecular crystal functional that is the result of a twofold solution to vdW-DF3's shortcomings. To obtain accurate binding energies we make use of vdW-DF3's flexible form of the non-local correlation, vital for the effective modeling of long-range dispersion interactions. In order to achieve accurate geometries we have also created a new variety of exchange in the generalized gradient approximation, one that allows for close fitting with experimental data. This new functional, which we name vdW-DF3-mc, outperforms all previously designed van der Waals density functionals and even the dispersion correction DFT-D3 in tests on molecular crystal data sets such as the X23. We also discuss how our optimization procedure can be applied to other types of systems, offering a broad range of applications.