Title: Dielectric continuum methods for quantum chemistry
Abstract
This review describes the theory and implementation of implicit solvation models based on continuum electrostatics. Within quantum chemistry this formalism is sometimes synonymous with the polarizable continuum model, a particular boundary‐element approach to the problem defined by the Poisson or Poisson–Boltzmann equation, but that moniker belies the diversity of available methods. This work reviews the current state‐of‐the art, with emphasis on theory and methods rather than applications. The basics of continuum electrostatics are described, including the nonequilibrium polarization response upon excitation or ionization of the solute. Nonelectrostatic interactions, which must be included in the model in order to obtain accurate solvation energies, are also described. Numerical techniques for implementing the equations are discussed, including linear‐scaling algorithms that can be used in classical or mixed quantum/classical biomolecular electrostatics calculations. Anisotropic models that can describe interfacial solvation are briefly described.
This article is categorized under:
Electronic Structure Theory > Ab Initio Electronic Structure Methods
Molecular and Statistical Mechanics > Free Energy Methods
Reliable simulations of molecules in condensed phase require the combination of an accurate quantum mechanical method for the core region, and a realistic model to describe the interaction with the environment. Additionally, this combination should not significantly increase the computational cost of the calculation compared to the corresponding in vacuo case. In this review, we describe the combination of methods based on coupled cluster (CC) theory with polarizable classical models for the environment. We use the polarizable continuum model (PCM) of solvation to discuss the equations, but we also show how the same theoretical framework can be extended to polarizable force fields. The theory is developed within the perturbation theory energy and singles‐T density (PTES) scheme, where the environmental response is computed with the CC single excitation amplitudes as an approximation for the full one‐particle reduced density. The CC‐PTES combination provides the best compromise between accuracy and computational effort for CC calculations in condensed phase, because it includes the response of the environment to the correlation density at the same computational cost of in vacuo CC. We discuss a number of numerical applications for ground and excited state properties, based on the implementation of CC‐PTES with single and double excitations (CCSD‐PTES), which show the reliability and computational efficiency of the method in reproducing experimental or full‐CC data.
This article is characterized under:
Electronic Structure Theory > Ab Initio Electronic Structure Methods
Electronic Structure Theory > Combined QM/MM Methods
Jurrus, Elizabeth; Engel, Dave; Star, Keith; Monson, Kyle; Brandi, Juan; Felberg, Lisa E.; Brookes, David H.; Wilson, Leighton; Chen, Jiahui; Liles, Karina; et al(
, Protein Science)
Abstract
The Adaptive Poisson–Boltzmann Solver (APBS) software was developed to solve the equations of continuum electrostatics for large biomolecular assemblages that have provided impact in the study of a broad range of chemical, biological, and biomedical applications. APBS addresses the three key technology challenges for understanding solvation and electrostatics in biomedical applications: accurate and efficient models for biomolecular solvation and electrostatics, robust and scalable software for applying those theories to biomolecular systems, and mechanisms for sharing and analyzing biomolecular electrostatics data in the scientific community. To address new research applications and advancing computational capabilities, we have continually updated APBS and its suite of accompanying software since its release in 2001. In this article, we discuss the models and capabilities that have recently been implemented within the APBS software package including a Poisson–Boltzmann analytical and a semi‐analytical solver, an optimized boundary element solver, a geometry‐based geometric flow solvation model, a graph theory‐based algorithm for determining pKavalues, and an improved web‐based visualization tool for viewing electrostatics.
Flavins have emerged as central to electron bifurcation, signaling, and countless enzymatic reactions. In bifurcation, two electrons acquired as a pair are separated in coupled transfers wherein the energy of both is concentrated on one of the two. This enables organisms to drive demanding reactions based on abundant low‐grade chemical fuel. To enable incorporation of this and other flavin capabilities into designed materials and devices, it is essential to understand fundamental principles of flavin electronic structure that make flavins so reactive and tunable by interactions with protein. Emerging computational tools can now replicate spectra of flavins and are gaining capacity to explain reactivity at atomistic resolution, based on electronic structures. Such fundamental understanding can moreover be transferrable to other chemical systems. A variety of computational innovations have been critical in reproducing experimental properties of flavins including their electronic spectra, vibrational signatures, and nuclear magnetic resonance (NMR) chemical shifts. A computational toolbox for understanding flavin reactivity moreover must be able to treat all five oxidation and protonation states, in addition to excited states that participate in flavoprotein's light‐driven reactions. Therefore, we compare emerging hybrid strategies and their successes in replicating effects of hydrogen bonding, the surrounding dielectric, and local electrostatics. These contribute to the protein's ability to modulate flavin reactivity, so we conclude with a survey of methods for incorporating the effects of the protein residues explicitly, as well as local dynamics. Computation is poised to elucidate the factors that affect a bound flavin's ability to mediate stunningly diverse reactions, and make life possible.
This article is categorized under:
Structure and Mechanism > Computational Biochemistry and Biophysics
Electronic Structure Theory > Combined QM/MM Methods
Williams‐Young, David B.; Petrone, Alessio; Sun, Shichao; Stetina, Torin F.; Lestrange, Patrick; Hoyer, Chad E.; Nascimento, Daniel R.; Koulias, Lauren; Wildman, Andrew; Kasper, Joseph; et al(
, WIREs Computational Molecular Science)
Abstract
The Chronus Quantum (ChronusQ) software package is an open source (under the GNU General Public License v2) software infrastructure which targets the solution of challenging problems that arise in ab initio electronic structure theory. Special emphasis is placed on the consistent treatment of time dependence and spin in the electronic wave function, as well as the inclusion of relativistic effects in said treatments. In addition, ChronusQ provides support for the inclusion of uniform finite magnetic fields as external perturbations through the use of gauge‐including atomic orbitals. ChronusQ is a parallel electronic structure code written in modern C++ which utilizes both message passing implementation and shared memory (OpenMP) parallelism. In addition to the examination of the current state of code base itself, a discussion regarding ongoing developments and developer contributions will also be provided.
This article is categorized under:
Software > Quantum Chemistry
Electronic Structure Theory > Ab Initio Electronic Structure Methods
Electronic Structure Theory > Density Functional Theory
Park, S. J.; Schwartz, B. J.(
, The journal of physical chemistry)
null
(Ed.)
Despite its importance in electron transfer reactions and radiation chemistry, there has been disagreement over the fundamental nature of the hydrated electron, such as whether or not it resides in a cavity. Mixed quantum/classical simulations of the hydrated electron give different structures depending on the pseudopotential employed, and ab initio models of computational necessity use small numbers of water molecules and/or provide insufficient statistics to compare to experimental observables. A few years ago, Kumar et al. (J. Phys. Chem. A 2015, 119, 9148) proposed a minimalist ab initio model of the hydrated electron with only a small number of explicitly treated water molecules plus a polarizable continuum model (PCM). They found that the optimized geometry had four waters arranged tetrahedrally around a central cavity, and that the calculated vertical detachment energy and radius of gyration agreed well with experiment, results that were largely independent of the level of theory employed. The model, however, is based on a fixed structure at 0 K and does not explicitly incorporate entropic contributions or the thermal fluctuations that should be associated with the room-temperature hydrated electron. Thus, in this paper, we extend the model of Kumar et al. by running Born−Oppenheimer molecular dynamics (BOMD) of a small number of water molecules with an excess electron plus PCM at room temperature. We find that when thermal fluctuations are introduced, the level of theory chosen becomes critical enough when only four waters are used that one of the waters dissociates from the cluster with certain density functionals. Moreover, even with an optimally tuned range-separated hybrid functional, at room temperature the tetrahedral orientation of the 0 K first-shell waters is entirely lost and the central cavity collapses, a process driven by the fact that the explicit water molecules prefer to make H-bonds with each other more than with the excess electron. The resulting average structure is quite similar to that produced by a noncavity mixed quantum/classical model, so that the minimalist 4-water BOMD models suffer from problems similar to those of noncavity models, such as predicting the wrong sign of the hydrated electron’s molar solvation volume. We also performed BOMD with 16 explicit water molecules plus an extra electron and PCM. We find that the inclusion of an entire second solvation shell of explicit water leads to little change in the outcome from when only four waters were used. In fact, the 16-water simulations behave much like those of water cluster anions, in which the electron localizes at the cluster surface, showing that PCM is not acceptable for use in minimalist models to describe the behavior of the bulk hydrated electron. For both the 4- and 16-water models, we investigate how the introduction of thermal motions alters the predicted absorption spectrum, vertical detachment energy, and resonance Raman spectrum of the simulated hydrated electron. We also present a set of structural criteria that can be used to numerically determine how cavity-like (or not) a particular hydrated electron model is. All of the results emphasize that the hydrated electron is a statistical object whose properties are inadequately captured using only a small number of explicit waters, and that a proper treatment of thermal fluctuations is critical to understanding the hydrated electron’s chemical and physical behavior.
@article{osti_10387064,
place = {Country unknown/Code not available},
title = {Dielectric continuum methods for quantum chemistry},
url = {https://par.nsf.gov/biblio/10387064},
DOI = {10.1002/wcms.1519},
abstractNote = {Abstract This review describes the theory and implementation of implicit solvation models based on continuum electrostatics. Within quantum chemistry this formalism is sometimes synonymous with the polarizable continuum model, a particular boundary‐element approach to the problem defined by the Poisson or Poisson–Boltzmann equation, but that moniker belies the diversity of available methods. This work reviews the current state‐of‐the art, with emphasis on theory and methods rather than applications. The basics of continuum electrostatics are described, including the nonequilibrium polarization response upon excitation or ionization of the solute. Nonelectrostatic interactions, which must be included in the model in order to obtain accurate solvation energies, are also described. Numerical techniques for implementing the equations are discussed, including linear‐scaling algorithms that can be used in classical or mixed quantum/classical biomolecular electrostatics calculations. Anisotropic models that can describe interfacial solvation are briefly described. This article is categorized under:Electronic Structure Theory > Ab Initio Electronic Structure MethodsMolecular and Statistical Mechanics > Free Energy Methods},
journal = {WIREs Computational Molecular Science},
volume = {11},
number = {4},
publisher = {Wiley Blackwell (John Wiley & Sons)},
author = {Herbert, John M.},
}
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