Leif D. Jacobson

Comment on "Does the Hydrated Electron Occupy a Cavity?"

Larsen et al. (Reports, 2 July 2010, p. 65) suggest that, contrary to the established paradigm, the aqueous electron does not carve out and occupy a cavity in liquid water. Closer examination of their theoretical model, however, reveals that many of its predictions differ substantively from established benchmarks and that its behavior differs qualitatively from Hartree-Fock theory, upon which the model is based.

An efficient, fragment-based electronic structure method for molecular systems: Self-consistent polarization with perturbative two-body exchange and dispersion

We report a fragment-based electronic structure method, intended for the study of clusters and molecular liquids, that incorporates electronic polarization (induction) in a self-consistent fashion but treats intermolecular exchange and dispersion interactions perturbatively, as post-self-consistent field corrections, using a form of pairwise symmetry-adapted perturbation theory. The computational cost of the method scales quadratically as a function of the number of fragments (monomers), but could be made to scale linearly by exploiting distance-dependent thresholds. Extensive benchmark calculations are reported using the S22 database of high-level ab initio binding energies for dimers, and we find that average errors can be reduced to <1 kcal/mol with a suitable choice of basis set. Comparison to ab initio benchmarks for water clusters as large as (H(2)O)(20) demonstrates that the method recovers ≳90% of the binding energy in these systems, at a tiny fraction of the computational cost. As such, this approach represents a promising path toward accurate, systematically improvable, and parameter-free simulation of molecular liquids.

A one-electron model for the aqueous electron that includes many-body electron-water polarization: Bulk equilibrium structure, vertical electron binding energy, and optical absorption spectrum

Previously, we reported an electron-water pseudopotential designed to be used in conjunction with a polarizable water model, in order to describe the hydrated electron [L. D. Jacobson et al., J. Chem. Phys. 130, 124115 (2009)]. Subsequently, we found this model to be inadequate for the aqueous electron in bulk water, and here we report a reparametrization of the model. Unlike the previous model, the current version is not fit directly to any observables; rather, we use an ab initio exchange-correlation potential, along with a repulsive potential that is fit to reproduce the density maximum of the excess electrons wave function within the static-exchange approximation. The new parametrization performs at least as well as the previous model, as compared to ab initio benchmarks for (H(2)O)(n) (-) clusters, and also predicts reasonable values for the diffusion coefficient, radius of gyration, and absorption maximum of the bulk species. The new model predicts a vertical electron binding energy of 3.7 eV in bulk water, which is 1.4 eV smaller than the value obtained using nonpolarizable models; the difference represents the solvents electronic reorganization energy following electron detachment. We find that the electrons first solvation shell is quite loose, which may be responsible for the electrons large, positive entropy of hydration. Many-body polarization alters the electronic absorption line shape in a qualitative way, giving rise to a high-energy tail that is observed experimentally but is absent in previous simulations. In our model, this feature arises from spatially diffuse excited states that are bound only by electronic reorganization (i.e., solvent polarization) following electronic excitation.

Nature's most squishy ion: The important role of solvent polarization in the description of the hydrated electron

The aqueous electron, e −(aq), and its finite analogues, the anionic water clusters , have attracted significant attention from both theory and experiment over the past few decades. Nevertheless, some of the most basic structural aspects of these systems, as well as the interpretation of certain spectroscopic features, remain controversial or else have defied theoretical explanation altogether. Due to the solvent-supported nature of the ion, a large number of water molecules is required in order to obtain a realistic model of e −(aq), and a wide variety of structural morphologies are available in clusters. These aspects severely limit the role of ab initio quantum chemistry in elucidating the properties of solvated-electron systems, but at the same time, the fundamentally quantum-mechanical nature of the ion must be taken into account. Most theoretical studies have therefore relied upon one-electron pseudopotential models and mixed quantum/classical molecular dynamics. In view of the highly diffuse, polarizable nature of the ion, however, it is surprising how little attention has been paid to the development of polarizable one-electron models. This article presents an overview of our efforts to develop such a model, as well as computational evidence to suggest that self-consistent, many-body electron–water polarization is qualitatively important in the description of both clusters and e −(aq) in bulk water.

Structure of the Aqueous Electron: Assessment of One-Electron Pseudopotential Models in Comparison to Experimental Data and Time-Dependent Density Functional Theory

The prevailing structural paradigm for the aqueous electron is that of an s-like ground-state wave function that inhabits a quasi-spherical solvent cavity, a viewpoint that is supported by numerous atomistic simulations using various one-electron pseudopotential models. This conceptual picture has recently been challenged, however, on the basis of results obtained from a new electron-water pseudopotential model that predicts a more delocalized wave function and no well-defined solvent cavity. Here, we examine this new model in comparison to two alternative, cavity-forming pseudopotential models. We find that the cavity-forming models are far more consistent with the experimental data for the electrons radius of gyration, optical absorption spectrum, and vertical electron binding energy. Calculations of the absorption spectrum using time-dependent density functional theory are in quantitative or semiquantitative agreement with experiment when the solvent geometries are obtained from the cavity-forming pseudopotential models, but differ markedly from experiment when geometries that do not form a cavity are used.

Polarization-Bound Quasi-Continuum States Are Responsible for the "Blue Tail" in the Optical Absorption Spectrum of the Aqueous Electron

The electronic absorption spectrum of the aqueous electron in bulk water has been simulated using long-range-corrected time-dependent density functional theory as well as mixed quantum/classical molecular dynamics based on a one-electron model in which electron-water polarization is treated self-consistently. Both methodologies suggest that the high-energy Lorentzian tail that is observed experimentally arises mostly from delocalized bound-state excitations of the electron rather than bound-to-continuum excitations, as is usually assumed. Excited states in the blue tail are bound only by polarization of the solvent electron density. These findings have potentially important ramifications for understanding electron localization in polar condensed media as well as biological radiation damage arising from dissociative electron attachment.

Automated Transition State Search and Its Application to Diverse Types of Organic Reactions

Transition state search is at the center of multiple types of computational chemical predictions related to mechanistic investigations, reactivity and regioselectivity predictions, and catalyst design. The process of finding transition states in practice is, however, a laborious multistep operation that requires significant user involvement. Here, we report a highly automated workflow designed to locate transition states for a given elementary reaction with minimal setup overhead. The only essential inputs required from the user are the structures of the separated reactants and products. The seamless workflow combining computational technologies from the fields of cheminformatics, molecular mechanics, and quantum chemistry automatically finds the most probable correspondence between the atoms in the reactants and the products, generates a transition state guess, launches a transition state search through a combined approach involving the relaxing string method and the quadratic synchronous transit, and finally validates the transition state via the analysis of the reactive chemical bonds and imaginary vibrational frequencies as well as by the intrinsic reaction coordinate method. Our approach does not target any specific reaction type, nor does it depend on training data; instead, it is meant to be of general applicability for a wide variety of reaction types. The workflow is highly flexible, permitting modifications such as a choice of accuracy, level of theory, basis set, or solvation treatment. Successfully located transition states can be used for setting up transition state guesses in related reactions, saving computational time and increasing the probability of success. The utility and performance of the method are demonstrated in applications to transition state searches in reactions typical for organic chemistry, medicinal chemistry, and homogeneous catalysis research. In particular, applications of our code to Michael additions, hydrogen abstractions, Diels-Alder cycloadditions, carbene insertions, and an enzyme reaction model involving a molybdenum complex are shown and discussed.

A Simple Algorithm for Determining Orthogonal, Self-Consistent Excited-State Wave Functions for a State-Specific Hamiltonian: Application to the Optical Spectrum of the Aqueous Electron.

We recently introduced a mixed quantum/classical model for the hydrated electron that includes electron/water polarization in a self-consistent fashion, using a polarizable force field for the water molecules [ J. Chem. Phys. 2010 , 133 , 154506 ]. Calculation of the electronic absorption spectrum for this model is not straightforward, owing to the state-specific nature of the Hamiltonian, the high density of electronic states, and the large solvent polarization response upon electronic excitation. Together, these properties make it difficult or impossible to converge the polarizable solvent dipoles self-consistently for each excited-state wave function. Here, we overcome this problem by means of an extended Lagrangian procedure for performing constrained annealing in the space of electronic variables. By construction, this algorithm affords self-consistent, mutually orthogonal solutions for any state-specific Hamiltonian, and we illustrate this approach by computing the optical spectrum of our polarizable model for the aqueous electron. The spectrum thus obtained affords better agreement with experiment than previous, perturbative calculations of solvent dipole relaxation. Strengths, weaknesses, and possible generalizations of this procedure are discussed.