Classical simulation of deposition of thiophene oligomers on TiO2-anatase: Relevance of long-range electrostatic interactions

We performed classical molecular dynamics simulations of the vapor-deposition of α-T4 oligomers on the TiO2-anatase (1 0 1) surface, comparing different sets of charges associated with the atoms of the model. The potential energy surfaces for α-T4 and TiO2 were described by re-parametrizations of the Universal force field with charges given by the charge equilibration (QEq) scheme, or with fixed charges obtained by an ab initio method using the Hirshfeld partition. The two sets of charges lead to completely different results for the interface formation, and for the characteristics of the organic film, with a clearly defined α-T4 contact layer in the QEq case, and a more homogeneous molecular distribution when using Hirshfeld charges. The main reason for the discrepancy was found to be the incorrect charge assignment given by QEq to the sulfur and α-carbon atoms in thiophenes, and highlight the relevance of long-range interactions in the organization of molecular films.