Binding energies of molecular solids from fragment and periodic approaches

Abstract

We calculate binding energies of four molecular solids using the Hartree-Fock (HF) and second-order Møller-Plesset perturbation theory (MP2). We obtain the energies within many-body expansion (MBE) as well as using periodic boundary conditions (PBC) to compare both approaches. The systems we study are methane, carbon dioxide, ammonia, and methanol. We use tight convergence settings to obtain the binding energies with a high precision, we estimate the uncertainties to be only few tenths of percent. We discuss several issues that affect the quality of the results and which need to be considered to reach high precision for both MBE and within PBC. For example, HF as well as MP2 energies within PBC benefited from the use of real-space Coulomb cut-off technique, the convergence of energies within MBE was improved by modifying the order of summation. Finally, numerical noise made the evaluation of some of the MBE contributions difficult and the effect was reduced by using smaller basis sets for the less critical terms.