191 research outputs found

    Triplet-Tuning: A Novel Family of Non-Empirical Exchange-Correlation Functionals

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    In the framework of DFT, the lowest triplet excited state, T1_1, can be evaluated using multiple formulations, the most straightforward of which are UDFT and TDDFT. Assuming the exact XC functional is applied, UDFT and TDDFT provide identical energies for T1_1 (ETE_{\rm T}), which is also a constraint that we require our XC functionals to obey. However, this condition is not satisfied by most of the popular XC functionals, leading to inaccurate predictions of low-lying, spectroscopically and photochemically important excited states, such as T1_1 and S1_1. Inspired by the optimal tuning strategy for frontier orbital energies [Stein, Kronik, and Baer, {\it J. Am. Chem. Soc.} {\bf 2009}, 131, 2818], we proposed a novel and non-empirical prescription of constructing an XC functional in which the agreement between UDFT and TDDFT in ETE_{\rm T} is strictly enforced. Referred to as "triplet tuning", our procedure allows us to formulate the XC functional on a case-by-case basis using the molecular structure as the exclusive input, without fitting to any experimental data. The first triplet tuned XC functional, TT-ω\omegaPBEh, is formulated as a long-range-corrected hybrid of PBE and HF functionals [Rohrdanz, Martins, and Herbert, {\it J. Chem. Phys.} {\bf 2009}, 130, 054112] and tested on four sets of large organic molecules. Compared to existing functionals, TT-ω\omegaPBEh manages to provide more accurate predictions for key spectroscopic and photochemical observables, including but not limited to ETE_{\rm T}, ESE_{\rm S}, ΔEST\Delta E_{\rm ST}, and II, as it adjusts the effective electron-hole interactions to arrive at the correct excitation energies. This promising triplet tuning scheme can be applied to a broad range of systems that were notorious in DFT for being extremely challenging

    Extended M{\o}ller-Plesset perturbation theory for dynamical and static correlations

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    We present a novel method that appropriately handles both dynamical and static electron correlation in a balanced manner, using a perturbation theory on a spin-extended Hartree-Fock (EHF) wave function reference. While EHF is a suitable candidate for degenerate systems where static correlation is ubiquitous, it is known that most of dynamical correlation is neglected in EHF. In this work, we derive a perturbative correction to a fully spin-projected self-consistent wave function based on second-order M{\o}ller-Plesset perturbation theory (MP2). The proposed method efficiently captures the ability of EHF to describe static correlation in degeneracy, combined with MP2's ability to treat dynamical correlation effects. We demonstrate drastic improvements on molecular ground state and excited state potential energy curves and singlet-triplet splitting energies over both EHF and MP2 with similar computational effort to the latter.Comment: 5 pages, 3 figures, 2 table

    Accurate densities of states for disordered systems from free probability: Live Free or Diagonalize

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    We investigate how free probability allows us to approximate the density of states in tight binding models of disordered electronic systems. Extending our previous studies of the Anderson model in neighbor interactions [J. Chen et al., Phys. Rev. Lett. 109, 036403 (2012)], we find that free probability continues to provide accurate approximations for systems with constant interactions on two- and three-dimensional lattices or with next-nearest-neighbor interactions, with the results being visually indistinguishable from the numerically exact solution. For systems with disordered interactions, we observe a small but visible degradation of the approximation. To explain this behavior of the free approximation, we develop and apply an asymptotic error analysis scheme to show that the approximation is accurate to the eighth moment in the density of states for systems with constant interactions, but is only accurate to sixth order for systems with disordered interactions. The error analysis also allows us to calculate asymptotic corrections to the density of states, allowing for systematically improvable approximations as well as insight into the sources of error without requiring a direct comparison to an exact solution

    Geometry of Molecular Motions in Dye Monolayers at Various Coverages

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    Molecular motion in monolayers is thought to influence the kinetics of charge transport and recombination in systems such as dye-sensitized solar cells (DSSCs). In this work, we use ab initio molecular dynamics to evaluate the geometry and time scale of such molecular motion in a D102 monolayer. D102, a dye that is routinely used in DSSCs, contains two chemical groups, namely, indoline and triphenylethylene, that are also present in many other dyes. We find that, at low surface coverage, the dye molecule exhibits two main tilting axes around which it heavily distorts within 10 ps. Further, the two benzene rings in the triphenylethylene group rotate with a 3–4-ps period. We observe that these large-amplitude movements are suppressed at full coverage, meaning that dye molecules in a monolayer are locked into place and undergo only minor conformational changes. Our observations indicate that, counterintuitively, charge diffusion across dye monolayers might be faster in the parts of the system that are characterized by a lower surface coverage. Because charge transport in dye monolayers has been shown to accelerate recombination kinetics in DSSCs, these results provide the basis for a new understanding of the electronic properties of sensitized systems and device efficiency.National Science Foundation (U.S.) (Grant CHE-1464804

    Quantum chemical approaches to [NiFe] hydrogenase

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    The mechanism by which [NiFe] hydrogenase catalyses the oxidation of molecular hydrogen is a significant yet challenging topic in bioinorganic chemistry. With far-reaching applications in renewable energy and carbon mitigation, significant effort has been invested in the study of these complexes. In particular, computational approaches offer a unique perspective on how this enzyme functions at an electronic and atomistic level. In this article, we discuss state-of-the art quantum chemical methods and how they have helped deepen our comprehension of [NiFe] hydrogenase. We outline the key strategies that can be used to compute the (i) geometry, (ii) electronic structure, (iii) thermodynamics and (iv) kinetic properties associated with the enzymatic activity of [NiFe] hydrogenase and other bioinorganic complexes

    Understanding the Dipole Moment of Liquid Water from a Self-Attractive Hartree Decomposition

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    The dipole moment of a single water molecule in liquid water has been a critical concept for understanding water’s dielectric properties. In this work, we investigate the dipole moment of liquid water through a self-attractive Hartree (SAH) decomposition of total electron density computed by density functional theory, on water clusters sampled from ab initio molecular dynamics simulation of bulk water. By adjusting one parameter that controls the degree of density localization, we reveal two distinct pictures of water dipoles that are consistent with bulk dielectric properties: a localized picture with smaller and less polarizable monomer dipoles and a delocalized picture with larger and more polarizable monomer dipoles. We further uncover that the collective dipole–dipole correlation is stronger in the localized picture and is key to connecting individual dipoles with bulk dielectric properties. On the basis of these findings, we suggest considering both individual and collective dipole behaviors when studying the dipole moment of liquid water and propose new design strategies for developing water models

    Understanding the Dipole Moment of Liquid Water from a Self-Attractive Hartree Decomposition

    Get PDF
    The dipole moment of a single water molecule in liquid water has been a critical concept for understanding water’s dielectric properties. In this work, we investigate the dipole moment of liquid water through a self-attractive Hartree (SAH) decomposition of total electron density computed by density functional theory, on water clusters sampled from ab initio molecular dynamics simulation of bulk water. By adjusting one parameter that controls the degree of density localization, we reveal two distinct pictures of water dipoles that are consistent with bulk dielectric properties: a localized picture with smaller and less polarizable monomer dipoles and a delocalized picture with larger and more polarizable monomer dipoles. We further uncover that the collective dipole–dipole correlation is stronger in the localized picture and is key to connecting individual dipoles with bulk dielectric properties. On the basis of these findings, we suggest considering both individual and collective dipole behaviors when studying the dipole moment of liquid water and propose new design strategies for developing water models
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