Publications

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Understanding charge transport processes at a molecular level is currently hindered by a lack of appropriate models for incorporating nonperiodic, anisotropic electric fields in molecular dynamics (MD) simulations. In this work, we develop a model for including electric fields in MD using an atomistic-to-continuum framework. This framework provides the mathematical and the algorithmic infrastructure to couple finite element (FE) representations of continuous data with atomic data. Our model represents the electric potential on a FE mesh satisfying a Poisson equation with source terms determined by the distribution of the atomic charges. Boundary conditions can be imposed naturally using the FE description of the potential, which then propagate to each atom through modified forces. The method is verified using simulations where analytical solutions are known or comparisons can be made to existing techniques. In addition, a calculation of a salt water solution in a silicon nanochannel is performed to demonstrate the method in a target scientific application in which ions are attracted to charged surfaces in the presence of electric fields and interfering media. © 2011 American Chemical Society.

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A rational approach was used to design polymeric materials for thin-film electronics applications, whereby theoretical modeling was used to determine synthetic targets. Time-dependent density functional theory calculations were used as a tool to predict the electrical properties of conjugated polymer systems. From these results, polymers with desirable energy levels and band-gaps were designed and synthesized. Measurements of optoelectronic properties were performed on the synthesized polymers and the results were compared to those of the theoretical model. From this work, the efficacy of the model was evaluated and new target polymers were identified.

Understanding charge transport processes at a molecular level using computational techniques is currently hindered by a lack of appropriate models for incorporating anisotropic electric fields, as occur at charged fluid/solid interfaces, in molecular dynamics (MD) simulations. In this work, we develop a model for including electric fields in MD using an atomistic-to-continuum framework. Our model represents the electric potential on a finite element mesh satisfying a Poisson equation with source terms determined by the distribution of the atomic charges. The method is verified using simulations where analytical solutions are known or comparisons can be made to existing techniques. A Calculation of a salt water solution in a silicon nanochannel is performed to demonstrate the method in a target scientific application.

The optoelectronic and excitonic properties in a series of linear acenes are investigated using range-separated methods within time-dependent density functional theory (TDDFT). In these highly-conjugated systems, we find that the range-separated formalism provides a substantially improved description of excitation energies compared to conventional hybrid functionals, which surprisingly fail for the various low-lying valence transitions. Moreover, we find that even if the percentage of Hartree-Fock exchange in conventional hybrids is re-optimized to match wavefunction-based CC2 benchmark calculations, they still yield serious errors in excitation energy trends. Based on an analysis of electron-hole transition density matrices, we also show that conventional hybrid functionals overdelocalize excitons and underestimate quasiparticle energy gaps in the acene systems. The results of the present study emphasize the importance of a range-separated and asymptotically-correct contribution of exchange in TDDFT for investigating optoelectronic and excitonic properties, even for these simple valence excitations.