Intermolecular charge transfer plays an important role in organic photovoltaics. However, the correct description of intermolecular charge transfer with density-functional theory has proven to be difficult because of the self-interaction error (SIE) in approximate exchange-correlation functionals.<br/>Here, we investigate a tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) model complex as a test case. We analyse the fractional charge error for this complex and find that the non-linearity of the monomer energies as a function of the total charge causes large errors in the derivative gap, which in turn leads to a negative HOMO-LUMO gap in the dimer.<br/>To overcome this error we apply subsystem density-functional theory. With this approach we partition the complex into two subsystems with opposite fractional charges and calculate the energy of the monomer subsystems and their interaction energy as a function of the fractional charge. For the monomer subsystems, we correct the fractional charge error by applying a linear interpolation between integer charges.<br/>We find that subsystem density-functional theory overcomes the SIE and describes the charge transfer correct. The transferred charge as a function of the electric field shows the expected integer electron jump.

Description of intermolecular charge transfer with subsystem density-functional theory

2019 Spring National Meeting

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