Abstract
The study of molecular solids has grown thanks to their significance as pharmaceuticals, organic semiconductor materials, and many other applications [1]. They can often have a rich phase diagram or exhibit polymorphism.
Polymorphic lattice energy differences are typically very small, on the level of one per cent of the lattice energies [2]. Reliable theoretical prediction of the binding energies of molecular solids and their polymorphs requires accurate methods, which faithfully describe all the interactions between the particles.
Many electronic structure techniques, including periodic density functional theory (DFT), periodic second-order Moller-Plesset perturbation theory (MP2), and diffusion Monte Carlo, have become an increasingly important tool for modeling molecular solids and polymorphism [3]. However, one issue of applying theoretical methods within periodic boundary conditions (PBC) for molecular solids is that there are several numerical parameters that affect the results, such as potentials, energy cutoff of the plane-wave basis set, and real space grid density.
Therefore, it is necessary to obtain converged binding energies of molecular solids with respect to these umerical parameters if one wants to understand the accuracy of given theoretical methods. Converging with these parameters increases significantly the overall cost of the calculations.
An alternative approach to obtain the binding energies of molecular solids is many-body expansion (MBE) which assembles the binding energy from contributions of 1-body, 2-body, 3-body and higher-order terms. Nevertheless, there are also some numerical parameters in this scheme, such as basis set, F12 setting, that need to be controlled to obtain converged binding energies of molecular solids.
In this study, we calculate the lattice energies of some small-molecule solids with varying physicochemical properties by using PBC and MBE approaches with DFT, random phase approximation (RPA) and MP2 methods to clarify some of the issues related to both approaches. The binding energies of molecular solids obtained from different methods are then compared with experimental values to identify the trend in their accuracy. [1] S.
L. Price, Chem.
Soc. Rev. 43, 2098 (2014) [2] J.
Nyman et al., CrystEngComm 17, 5154 (2015) [3] G. J.
O. Beran, Chem.
Rev. 116, 5567 (2016)