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    Achieving Predictive Description of Negative Differential Resistance in Molecular Junctions Using a Range-Separated Hybrid Functional
    (Wiley-VCH Verlag, 2021) Bhandari, Srijana; Yamada, Atsushi; Hoskins, Austin; Payne, Jameson; Aksu, Huseyin; Dunietz, Barry D.
    Range-separated hybrid (RSH) functionals have been recently used to overcome the tendency of traditional density functional theory (DFT) calculations to overestimate the conductance of molecular junctions. Non-equilibrium conditions are addressed following non-equilibrium Green's function (NEGF) formulation with RSH functionals to study negative differential resistance (NDR) in molecular junctions of oligo phenylene ethylene derivatives linking gold electrodes. It is shown that the RSH-NEGF calculations indicate NDR onset bias that agrees well with measured trends, associate NDR to orbital localization at the drain contact, and analyze the role of junction asymmetry in NDR. The RSH-NEGF results are also compared with alternative DFT-NEGF combinations to highlight the importance of basing the computational study on a functional that achieves physically significant frontier orbitals. Finally, the effects of thermally accessible molecular fluctuations to enhance the NDR conductance drop are also discussed.
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    CTRAMER: An open-source software package for correlating interfacial charge transfer rate constants with donor/acceptor geometries in organic photovoltaic materials
    (American Institute of Physics Inc., 2021) Tinnin, Jacob; Aksu, Huseyin; Tong, Zhengqing; Zhang, Pengzhi; Geva, Eitan; Dunietz, Barry D.; Sun, Xiang
    In this paper, we present CTRAMER (Charge-Transfer RAtes from Molecular dynamics, Electronic structure, and Rate theory)-an open-source software package for calculating interfacial charge-transfer (CT) rate constants in organic photovoltaic (OPV) materials based on ab initio calculations and molecular dynamics simulations. The software is based on identifying representative donor/acceptor geometries within interfacial structures obtained from molecular dynamics simulation of donor/acceptor blends and calculating the corresponding Fermi's golden rule CT rate constants within the framework of the linearized-semiclassical approximation. While the methods used are well established, the integration of these state-of-the-art tools originating from different disciplines to study photoinduced CT processes with explicit treatment of the environment, in our opinion, makes this package unique and innovative. The software also provides tools for investigating other observables of interest. After outlining the features and implementation details, the usage and performance of the software are demonstrated with results from an example OPV system.
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    How Well Does a Solvated Octa-acid Capsule Shield the Embedded Chromophore? A Computational Analysis Based on an Anisotropic Dielectric Continuum Model
    (Amer Chemical Soc, 2020) Aksu, Huseyin; Paul, Suranjan K.; Herbert, John M.; Dunietz, Barry D.
    The optical properties of chromophores embedded in a water-solvated dimer of octa-acid that forms a molecular-shaped capsule are investigated. In particular, we address the anisotropic dielectric environment that appears to blue-shift excitation energies compared to the free aqueous chromophores. Recently we reported that using an effective scalar dielectric constant epsilon approximate to 3 appears to reproduce the measured spectra of the embedded coumarins, suggesting that the capsule provides a significant, albeit not perfect, screening of the aqueous dielectric environment. Here, we report absorption energies using a theoretical treatment that includes continuum solvation affected by an anisotropic dielectric function reflecting the high-dielectric environment outside of the capsule and the low-dielectric region within. We report time-dependent density functional theory calculations using a range-separated functional with the Poisson boundary conditions that model the anisotropic dielectric environment. Our calculations find that the anisotropic environment due to the water-solvated hydrophobic capsule is equivalent to a homogeneous effective dielectric constant of approximate to 3. The calculated values also appear to reproduce measured absorption of the embedded coumarin, where we study the effect of the hydrophobic capsule on the excited state.
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    On the Interplay between Electronic Structure and Polarizable Force Fields When Calculating Solution-Phase Charge-Transfer Rates
    (Amer Chemical Soc, 2020) Han, Jaebeom; Zhang, Pengzhi; Aksu, Huseyin; Maiti, Buddhadev; Sun, Xiang; Geva, Eitan; Dunietz, Barry D.
    We present a comprehensive analysis of the interplay between the choice of an electronic structure method and the effect of using polarizable force fields vs. nonpolarizable force fields when calculating solution-phase charge-transfer (CT) rates. The analysis is based on an integrative approach that combines inputs from electronic structure calculations and molecular dynamics simulations and is performed in the context of the carotenoid-porphyrin-C-60 molecular triad dissolved in an explicit tetrahydrofuran (THF) liquid solvent. Marcus theory rate constants are calculated for the multiple CT processes that occur in this system based on either polarizable or nonpolarizable force fields, parameterized using density functional theory (DFT) with either the B3LYP or the Baer-Neuhauser-Livshits (BNL) density functionals. We find that the effect of switching from nonpolarizable to polarizable force fields on the CT rates is strongly dependent on the choice of the density functional. More specifically, the rate constants obtained using polarizable and nonpolarizable force fields differ significantly when B3LYP is used, while much smaller changes are observed when BNL is used. It is shown that this behavior can be traced back to the tendency of B3LYP to overstabilize CT states, thereby pushing the underlying electronic transitions to the deep inverted region, where even small changes in the force fields can lead to significant changes in the CT rate constants. Our results demonstrate the importance of combining polarizable force fields with an electronic structure method that can accurately capture the energies of excited CT states when calculating charge-transfer rates.
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    Photoinduced charge transfer in Zn(II) and Au(III)-ligated symmetric and asymmetric bacteriochlorin dyads: A computational study
    (Amer Inst Physics, 2020) Aksu, Huseyin; Maiti, Buddhadev; Ptaszek, Marcin; Dunietz, Barry D.
    The excited-state properties and photoinduced charge-transfer (CT) kinetics in a series of symmetrical and asymmetrical Zn- and Au-ligated meso-meso-connected bacteriochlorin (BChl) complexes are studied computationally. BChl derivatives, which are excellent near-IR absorbing chromophores, are found to play a central role in bacterial photosynthetic reaction centers but are rarely used in artificial solar energy harvesting systems. The optical properties of chemically linked BChl complexes can be tuned by varying the linking group and involving different ligated metal ions. We investigate charge transfer in BChl dyads that are either directly linked or through a phenylene ring (1,4-phenylene) and which are ligating Zn or Au ions. The directly linked dyads with a nearly perpendicular arrangement of the BChl units bear markedly different properties than phenylene linked dyads. In addition, we find that the dielectric dependence of the intramolecular CT rate is very strong in neutral Zn-ligated dyads, whereas cationic Au-ligated dyads show negligible dielectric dependence of the CT rate. Rate constants of the photo induced CT process are calculated at the semiclassical Marcus level and are compared to fully quantum mechanical Fermi's golden rule based values. The rates are calculated using a screened range separated hybrid functional that offers a consistent framework for addressing environment polarization. We study solvated systems in two solvents of a low and a high scalar dielectric constant.
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    Second-order Brillouin-Wigner perturbation theory: size-extensivity correction
    (Springer, 2012) Aksu, Huseyin
    Size-extensivity correction to second-order Brillouin-Wigner perturbation theory is derived and implemented. For the size-extensivity correction, renormalization term is obtained by modifying the non-physical term in the denominator of the second-order Brillouin-Wigner formulae. We have achieved improved results. The method is illustrated in a series calculations on H2O, BH, HOF, Be and Ne using three different basis sets, and the results are then compared with corresponding coupled cluster double and second-order MOller-Plesset perturbation theory.