192 research outputs found
Triplet-Tuning: A Novel Family of Non-Empirical Exchange-Correlation Functionals
In the framework of DFT, the lowest triplet excited state, T, 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 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 T and S. 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 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-PBEh, 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-PBEh manages to provide more
accurate predictions for key spectroscopic and photochemical observables,
including but not limited to , , , and
, 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
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
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
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Towards Prediction of Non-Radiative Decay Pathways in Organic Compounds I: The Case of Naphthalene Quantum Yields
Many emerging technologies depend on human’s ability to control and manipulate the excited-state properties of molecular systems. These technologies include fluorescent labeling in biomedical imaging, light harvesting in photovoltaics, and electroluminescence in light-emitting devices. All of these systems suffer from non-radiative loss pathways that dissipate electronic energy as heat, which causes the overall system efficiency to be directly linked to quantum yield (Φ) of the molecular excited state. Unfortunately, Φ is very difficult to predict from first principles because the description of a slow non-radiative decay mechanism requires an accurate description of long-timescale excited-state quantum dynamics. In the present study, we introduce an efficient semiempirical method of calculating the fluorescence quantum yield (Φfl) for molecular chromophores, which, based on machine learning, converts simple electronic energies computed using time-dependent density functional theory (TDDFT) into an estimate of Φfl. As with all machine learning strategies, the algorithm needs to be trained on fluorescent dyes for which Φfl’s are known, so as to provide a black-box method which can later predict Φfl’s for chemically similar chromophores that have not been studied experimentally. As a first illustration of how our proposed algorithm can be trained, we examine a family of 25 naphthalene derivatives. The simplest application of the energy gap law is found to be inadequate to explain the rates of internal conversion (IC) or intersystem crossing (ISC) – the electronic properties of at least one higher-lying electronic state (Sn or Tn) or one far-from-equilibrium geometry are typically needed to obtain accurate results. Indeed, the key descriptors turn out to be the transition state between the Franck–Condon minimum a distorted local minimum near an S0/S1 conical intersection (which governs IC) and the magnitude of the spin–orbit coupling (which governs ISC). The resulting Φfl’s are predicted with reasonable accuracy (±22%), making our approach a promising ingredient for high-throughput screening and rational design of the molecular excited states with desired Φ’s. We thus conclude that our model, while semi-empirical in nature, does in fact extract sound physical insight into the challenge of describing non-radiative relaxations
Geometry of Molecular Motions in Dye Monolayers at Various Coverages
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
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
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
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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