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Coto P.B.,Friedrich - Alexander - University, Erlangen - Nuremberg | Sharifzadeh S.,Lawrence Berkeley National Laboratory | Neaton J.B.,Lawrence Berkeley National Laboratory | Neaton J.B.,University of California at Berkeley | And 2 more authors.
Journal of Chemical Theory and Computation | Year: 2015

The lowest-lying electronic excited states of pentacene and its oligomers are investigated using accurate multireference wave function methods (CASPT2/CASSCF) and the many-body Greenss function approach (GW/BSE). The results obtained for dimers and trimers of different geometry reveal a complex electronic structure, which includes locally excited, charge transfer, and multiexciton states. For singlets of single-excitation character, both approaches yield excitation energies that are in good overall quantitative agreement. While the multiexciton states are located relatively high in energy in all systems investigated, charge transfer states exist in close proximity to the lowest-lying absorbing states. The implications of the results for the mechanisms of singlet fission in pentacene are discussed. © 2014 American Chemical Society.

Liu Z.-F.,Lawrence Berkeley National Laboratory | Wei S.,Columbia University | Yoon H.,Yonsei University | Adak O.,Columbia University | And 8 more authors.
Nano Letters | Year: 2014

Using scanning tunneling microscope break-junction experiments and a new first-principles approach to conductance calculations, we report and explain low-bias charge transport behavior of four types of metal-porphyrin-gold molecular junctions. A nonequilibrium Green's function approach based on self-energy corrected density functional theory and optimally tuned range-separated hybrid functionals is developed and used to understand experimental trends quantitatively. Importantly, due to the localized d states of the porphyrin molecules, hybrid functionals are essential for explaining measurements; standard semilocal functionals yield qualitatively incorrect results. Comparing directly with experiments, we show that the conductance can change by nearly a factor of 2 when different metal cations are used, counter to trends expected from gas-phase ionization energies which are relatively unchanged with the metal center. Our work explains the sensitivity of the porphyrin conductance with the metal center via a detailed and quantitative portrait of the interface electronic structure and provides a new framework for understanding transport quantitatively in complex junctions involving molecules with localized d states of relevance to light harvesting and energy conversion. © 2014 American Chemical Society.

Tamblyn I.,University of Ontario Institute of Technology | Refaely-Abramson S.,Weizmann Institute of Science | Neaton J.B.,Lawrence Berkeley National Laboratory | Neaton J.B.,University of California at Berkeley | And 2 more authors.
Journal of Physical Chemistry Letters | Year: 2014

A self-consistent optimally tuned range-separated hybrid density functional (scOT-RSH) approach is developed. It can simultaneously predict accurate geometries, vibrational modes, and frontier orbital energies. This is achieved by optimizing the range-separation parameter, γ to both satisfy the ionization energy theorem and minimize interatomic forces. We benchmark our approach against an established hybrid functional, B3LYP, using the G2 test set. scOT-RSH greatly improves the accuracy of occupied frontier orbital energies, with a mean absolute error (MAE) of only 0.2 eV relative to experimental ionization energies compared to 2.96 eV with B3LYP. Geometries do not change significantly compared to those obtained from B3LYP, with a bond length MAE of 0.012 Å compared to 0.008 Å for B3LYP, and a 6.5% MAE for zero-point energies, slightly larger than that of B3LYP (3.1%). scOT-RSH represents a new paradigm in which accurate geometries and ionization energies can be predicted simultaneously from a single functional approach. © 2014 American Chemical Society.

Poloni R.,CNRS Materials Science and Engineering | Poloni R.,University of California at Berkeley | Poloni R.,Lawrence Berkeley National Laboratory | Lee K.,University of California at Berkeley | And 6 more authors.
Journal of Physical Chemistry Letters | Year: 2014

Using van der Waals-corrected density functional theory and a local chemical bond analysis, we study and explain trends in the binding between CO2 and open-metal coordination sites within a series of two metal-organic frameworks (MOFs), BTT, and MOF-74 for Ca, Mg, and nine divalent transition-metal cations. We find that Ti and V result in the largest CO 2 binding energies and show that for these cations the CO2 binding energies for both structure types are twice the value expected based on pure electrostatics. We associate this behavior with the specific electronic configuration of the divalent cations and symmetry of the metal coordination site upon CO2 binding, which result in empty antibonding orbitals between CO2 and the metal cation. We demonstrate that a chemical bond analysis and electrostatic considerations can be used to predict trends of CO2 binding affinities to MOFs with transition-metal cations. © 2014 American Chemical Society.

Egger D.A.,Weizmann Institute of Science | Liu Z.-F.,Lawrence Berkeley National Laboratory | Neaton J.B.,Lawrence Berkeley National Laboratory | Neaton J.B.,University of California at Berkeley | And 2 more authors.
Nano Letters | Year: 2015

A key quantity for molecule-metal interfaces is the energy level alignment of molecular electronic states with the metallic Fermi level. We develop and apply an efficient theoretical method, based on density functional theory (DFT) that can yield quantitatively accurate energy level alignment information for physisorbed metal-molecule interfaces. The method builds on the "DFT+Σ" approach, grounded in many-body perturbation theory, which introduces an approximate electron self-energy that corrects the level alignment obtained from conventional DFT for missing exchange and correlation effects associated with the gas-phase molecule and substrate polarization. Here, we extend the DFT+Σ approach in two important ways: first, we employ optimally tuned range-separated hybrid functionals to compute the gas-phase term, rather than rely on GW or total energy differences as in prior work; second, we use a nonclassical DFT-determined image-charge plane of the metallic surface to compute the substrate polarization term, rather than the classical DFT-derived image plane used previously. We validate this new approach by a detailed comparison with experimental and theoretical reference data for several prototypical molecule-metal interfaces, where excellent agreement with experiment is achieved: benzene on graphite (0001), and 1,4-benzenediamine, Cu-phthalocyanine, and 3,4,9,10-perylene-tetracarboxylic-dianhydride on Au(111). In particular, we show that the method correctly captures level alignment trends across chemical systems and that it retains its accuracy even for molecules for which conventional DFT suffers from severe self-interaction errors. (Figure Presented). © 2015 American Chemical Society.

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