Jürgen Schnitker

Quantum simulation study of the hydrated electron

An excess electron in a sample of classical water molecules at room temperature has been simulated using path integral techniques. The electron–water interaction is modeled by a pseudopotential with effective core repulsion and further terms for the Coulomb interaction and polarization effects. Various discretizations of the electron path, up to 1000 points, are examined. The charge distribution of the electron is found to be compact and to occupy a cavity in the water, in agreement with the conventional picture. The solvation shell structure is similar to that of relatively large solvated atomic anions, but the radial electron‐solvent correlations are largely smeared out due to fluctuations of the electronic density distribution. In parts of the simulation the structure of the first solvation shell corresponds on the average to the structure proposed for hydrated electrons by Kevan. The computed solvation energy and the estimated energy of the first optical excitation agree reasonably well with experimen...

An electron–water pseudopotential for condensed phase simulation

A simple electron–molecule pseudopotential is obtained that describes the interaction between an excess electron and a rigid water molecule in the electronic ground state. The potential is completely local and involves only spherically symmetric terms with respect to the three molecular nuclei (interaction site model). The potential is thus suitable for large‐scale computer simulations, as well as more analytical theories. A description is given of the contributions included in this potential, as well as the ramifications of alternative choices.

Model dependence of quantum isotope effects in liquid water

Path‐integral molecular‐dynamics simulations have been carried out for liquid water at room temperature using three different potential functions: ST2, SPC, and TIP4P. Quantum isotope effects on the liquid structure are computed in order to examine the dependence of these structural changes on the model used, and a comparison is made to corresponding measurements. The SPC model is found to be in excellent agreement with experimental results; for this model the oxygen–oxygen pair distribution function shows a change in shape and slight shift to smaller distance of the second‐neighbor peak when going from D2O to H2O. In contrast, the other two models both show a distinct outward shift of this peak. This difference between models can be attributed to subtle differences in the direct interaction energies of second‐nearest‐neighbor molecules.

Electron localization in liquid water: A computer simulation study of microscopic trapping sites

Sites at which excess electrons in pure water at 10 °C could become initially localized and subsequently rapidly solvated are identified by scanning a molecular dynamics simulation of pure equilibrium ST2 water with a test charge. Very favorable sites with respect to both volume and electrostatic potential are found in relatively high number density. Although the observed distribution of large cavities spans a wide range with respect to the electrostatic potential at the cavity centers, the calculated potential energy at the most favorable sites is almost as large as the apparent hydration energy of the electron in the equilibrated state.

Transient photophysical hole‐burning spectroscopy of the hydrated electron: A quantum dynamical simulation

Results for the time‐dependent adiabatic eigenspectrum of an electron in water evolving in dynamic equilibrium have been obtained via quantum molecular dynamics simulation and used to evaluate the results expected from time‐resolved transient optical hole‐burning experiments. The dependence on excitation frequency and pulse length have been explored. The calculated results indicate that a relatively broad hole is created, but that, for ultrashort pump–probe time delays (≤100 fs) and comparably short pulses, the shape is distinctly different from the equilibrium spectrum. A slower component in the spectral evolution is also present, but appears likely to be difficult to distinguish experimentally. The shape of the absorption deficit is characteristic of the inhomogeneously broadened 1s, 2p‐type electronic state structure found previously to underlie the equilibrium spectrum, and distinguishes between this description and a number of proposed alternatives. With pulse durations comparable to the best now ava...

A comparison of classical and quantum analyses of electron localization sites in liquid water

The results of an earlier study [Schnitker, Rossky, and Kenney‐Wallace, J. Chem. Phys. 85, 2986 (1986)] in which likely sites for electron localization in pure liquid water were identified and characterized via a physically motivated purely classical analysis are statistically compared to a corresponding fully quantum mechanical treatment of the excess electronic ground state. It is shown that the most energetically favorable localization sites identified by the classical treatment correspond reasonably to the quantum mechanical result both energetically and spatially. It is found that the existence and location of a physically localized ground state can be determined from the classical results if both the minimum of the estimated absolute total electronic energy and the difference between this minimum and the alternative local minima identified within a solvent configuration are considered. Further, the results confirm that the concentration of such effective sites is relatively high in the liquid (∼0.01 M). Hence, the classical approach has merit as a qualitative tool for the analysis of the electronic states supported by the preexisting configurational order in a liquid.

Quantum simulations of aqueous systems

Discretized path-integral simulation methods have been applied to the determination of structure in two quantum mechanical aqueous systems. The first of these applications is the determination of the consequences of quantizing the rigid-body degrees of freedom of the water molecules in the many-particle pure room temperature liquid. The results provide a quantitative estimate of the significance of approximating such a system as classical and also of the size of isotope effects on the liquid structure. These features are found to have a close analogue in the structural response of the fluid to temperature. Second, we consider the structure of a hydrated excess electron. Here we treat the water classically but treat the highly quantum mechanical electron via a path-integral description, introducing a local electron-water pseudopotential for the interaction. The excess electron density and solvent distribution are examined and shown to exhibit strong structural similarities to ionic solvation. However, it is found that the electronic density fluctuates sufficiently in size and shape as to nearly erase distinct features in the electron-solvent radial correlations. For both of the aqueous systems considered, comparison of results following from the simulations with experimentally accessible direct structural measures yields satisfactory agreement.

Response to “Comment on ‘An electron-water pseudopotential for condensed phase simulation’ ” [J. Chem. Phys. 131, 037101 (2009)]

We are grateful for the detailed re-examination of Larsen et al. of the electron-water pseudopotential that we proposed more than 20 years ago. The potential was among the first of a number of potentials that were used by various groups in studies of the structure, dynamics, and spectroscopy of solvated electrons. While our potential was based on a more detailed analysis than had been carried out in previous work, it nevertheless included many assumptions and strong approximations. Effectively, it was an ad hoc model potential—irrespective of its construction from basic physical arguments. It has been validated, at least on a qualitative level, by comparison to experiment in a number of contexts, most importantly excited state dynamics. When devising the potential, we considered four fundamental contributions: i static Coulomb interactions between the excess electron and the dipolar water molecules, ii electrostatic polarization effects, iii Pauli repulsion reflecting the orthogonality constraint between the excess electronic wave function and the solvent wave functions, and iv exchange interactions. The last were omitted from the final potential, but the other three terms were deemed significant enough to be included. As Larsen et al. showed, an error was apparently made in the calculation of the parameters that are associated with one of the three terms that are represented in the potential. This means that our model potential does not, in fact, follow from the procedure as originally described. Effectively, a correct calculation following the route originally proposed shifts the balance in favor of the repulsive orthogonality term, correspondingly de-emphasizing the Coulomb interaction with the polar water molecules. However, what Larsen et al. observed when simulations are carried out with the “corrected” potential is remarkable. Despite the considerable change in water distribution at a short distance from the electron, once again a cavity-like state for the hydrated electron is found—just as has been seen in numerous other studies of hydrated electrons that employed alternative potential functions. There are some differences, in that the relatively smaller contribution of the polar interaction term leads to a solvation structure that is considerably less akin to that of an anion than was observed in the original simulations. One would then conclude that not only anions but also small nonpolar solutes can provide a meaningful reference point for the description of the hydrated electron. The calculated absorption spectrum is found to be redshifted, albeit without any real change in the lineshape or the underlying origin of that lineshape. While the spatial details of the hydration structure of hydrated electrons remain unsettled, we believe that the comparison of Larsen et al. of the two potential functions only confirms the general robustness of our understanding that has emerged over the past two decades: There is a cavity with an electronic ground state that can be s-like, and the main band of the absorption spectrum can be explained by transitions to three excited states that can be p-like and that are only approximately degenerate. Simulations of hydrated electrons with sophisticated many-electron methods indicate that the precise description of the hydrated electron may be more involved, but the simple cavity model nevertheless continues to serve as a most useful reference point.