Importance of London dispersion effects for the packing of molecular crystals: a case study for intramolecular stacking in a bis-thiophene derivative

The influence of London dispersion effects in density functional theory for the computation of molecular crystals is investigated. The structure of dithienobicyclo[4.4.1]-undeca-3,8-dien-11-one ethylene glycol ketal (RESVAN) that has been studied previously under isolated molecule conditions is considered as an example. The intramolecular stacking of the two thiophene rings is strongly influenced by non-covalent interactions. Previous investigations revealed significant deviations between computed molecular and experimentally observed crystal sulfur-sulfur distances. The influence of the crystal environment on this distance is estimated by calculating potential energy curves for the gas phase'' and in the crystal using the periodic plane-wave code VASP. With three common dispersion corrected (DFT-D3) density functionals (PBE, revPBE and PBEsol), an average shortening of the sulfur-sulfur distance by 0.10 angstrom (packing'' effect) is computed which leads to a corrected experimental'' gas-phase value of R(S-S) = 4.19 angstrom. This value is in good agreement with the results of reliable quantum chemical methods and serves as a challenging benchmark for electronic structure methods. Overall the crystal environment leads to a widening of the molecular structure due to intermolecular dispersion interaction. This is opposite to uncorrected DFT that suggest a compression due to Pauli exchange repulsion. In the crystal the intra-and inter-molecular dispersion effects are large but of opposite sign and their effect almost cancels out so that in this example dispersion-uncorrected DFT yields reasonable solid state structures. This is different for the computation of the lattice energy for which reasonable values are obtained only at the DFT-D3 level.