Constrained Density Functional Theory of Molecular Dimers

For charge transport in organic semiconductors the geometrical response to the presence of the charge plays a crucial role. Often, charge transport in these materials can be considered as the hopping of a localized polaron. Unfortunately, the description of localized charge carriers within semilocal Density Functional Theory (DFT) is prevented by the self-interaction error that artificially delocalizes the charge. Here, we present a computational scheme for the description of localized charges in an organic semiconductor. Constrained DFT is used to localize the charge on one of the molecules of a molecular dimer. The availability of the forces from this constraint enables ab initio molecular dynamics calculations and gives access to the geometrical response of neighboring molecules to the presence of a charged neighbor. This is demonstrated for a pentacene dimer. The reorganization energy is found to increase from 91 meV to 108 meV when decreasing the distance between two Pentacene molecules from 7 Å to 4 Å.