Description of Intermolecular Charge Transfer With Subsystem Density-Functional Theory

22 August 2019, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


Efficient quantum-chemical methods that are able to describe intermolecular charge are crucial for modeling organic semiconductors. However, the correct description of intermolecular charge transfer with density-functional theory (DFT) is hampered by the fractional charge error of approximate exchange-correlation (xc) functionals. Here, we investigate the charge transfer induced by an external electric field in a tetrathiafulvalene--tetracyanoquinodimethane (TTF--TCNQ) complex as a test case. For this seemingly simple model system, a supermolecular DFT treatment fails with most conventional xc functionals. Here, we present an extension of subsystem DFT to subsystems with a fractional number of electrons. We show that within such a framework it becomes possible to overcome the fractional charge error by enforcing the correct dependence of each subsystem's total energy on the subsystem's fractional charge. Such a subsystem DFT approach allows for a correct description of the intermolecular charge transfer in the TTF--TCNQ model complex. The approach presented here can be generalized to larger molecular aggregates and will thus allow for modeling organic semiconductor materials accurately and efficiently.


DFT, Density Functional Theory
fractional charge error
charge transfer
organic semiconductor materials

Supplementary materials



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