Density functional theory (DFT) is the method of choice for quantum mechanically computing the energies and properties of large systems but the lack of nonlocal exact exchange leads to over-delocalization and over-polarization of the system. For a set of linearly increasing alkanes, alkenes and oligoacenes as well as a series of non-interacting organic molecules (methanes, ethenes, and benzenes), I will show that there is a size-dependent error in the DFT computed ionization potential (IP). In addition, I will present results for the IP in aqueous solution using a polarizable continuum model, a molecular mechanical point charge model, and a quantum mechanical (QM) treatment of explicit solvent. The underestimated IP is due to the removed electron density being spread across multiple molecules instead of only from one. Therefore, the size-dependent IP error increases when the number of explicit solvent molecules in the QM region increases.<br/>The addition of a significant amount of exact exchange improves the delocalization error and accurately describes the removal of the electron from just one molecule. Range separated hybrid (RSH) functionals split exchange into short- and long-range components, allowing DFT approximate exchange to be used at short-range and exact exchange at long-range. The range separation parameter can be tuned to adjust the amount of exact exchange in the system which can greatly improve the deficiencies of DFT. In vacuum and explicit solvent, this tuned range separation parameter represents the point where the density of the removed electron becomes localized.

Size-dependent error of the density-functional theory ionization potential in vacuum and solution

251st National Meeting (2016)

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