53 research outputs found
Nuclear quantum effects on the non-adiabatic decay mechanism of an excited hydrated electron
We present a kinetic analysis of the non-adiabatic decay mechanism of an excited state hydrated electron to the ground state. The theoretical treatment is based on a quantized, gap dependent golden rule rate constant formula which describes the non-adiabatic transition rate between two quantum states. The rate formula is expressed in terms of quantum time correlation functions of the energy gap, and of the non-adiabatic coupling. These gap dependent quantities are evaluated from three different sets of mixed quantum-classical molecular dynamics simulations of a hydrated electron equilibrated a) in its ground state, b) in its first excited state, and c) on a hypothetical mixed potential energy surface which is the average of the ground and the first excited electronic states. The quantized, gap-dependent rate results are applied in a phenomenological kinetic equation which provides the survival probability function of the excited state electron. Although the lifetime of the equilibrated excited state electron is computed to be very short (well under 100 fs), the survival probability function for the non-equilibrium process in pump-probe experiments yields an effective excited state lifetime of around 300 fs, a value consistent with the findings of several experimental groups and previous theoretical estimates
Nuclear quantum effects in electronically adiabatic quantum time correlation functions : Application to the absorption spectrum of a hydrated electron
A general formalism for introducing nuclear quantum effects in the expression of the quantum time correlation function of an operator in a multi-level electronic system is presented in the adiabatic limit. The final formula includes the nuclear quantum time correlation functions of the operator matrix elements, of the energy gap, and their cross terms. These quantities can be inferred and evaluated from their classical analogs obtained by mixed quantum-classical molecular dynamics simulations. The formalism is applied to the absorption spectrum of a hydrated electron, expressed in terms of the time correlation function of the dipole operator in the ground electronic state. We find that both static and dynamic nuclear quantum effects distinctly influence the shape of the absorption spectrum, especially its high-energy tail related to transitions to delocalized electron states. Their inclusion does improve significantly the agreement between theory and experiment for both the low and high frequency edges of the spectrum. It does not appear sufficient, however, to resolve persistent deviations in the slow Lorentzian-like decay part of the spectrum in the intermediate 2-3 eV region
Fast Computation of Solvation Free Energies with Molecular Density Functional Theory: Thermodynamic-Ensemble Partial Molar Volume Corrections
Molecular Density Functional Theory (MDFT) offers an efficient implicit-
solvent method to estimate molecule solvation free-energies whereas conserving
a fully molecular representation of the solvent. Even within a second order ap-
proximation for the free-energy functional, the so-called homogeneous reference
uid approximation, we show that the hydration free-energies computed for a
dataset of 500 organic compounds are of similar quality as those obtained from
molecular dynamics free-energy perturbation simulations, with a computer cost
reduced by two to three orders of magnitude. This requires to introduce the
proper partial volume correction to transform the results from the grand
canoni- cal to the isobaric-isotherm ensemble that is pertinent to experiments.
We show that this correction can be extended to 3D-RISM calculations, giving a
sound theoretical justifcation to empirical partial molar volume corrections
that have been proposed recently.Comment: Version with correct equation numbers is here:
http://compchemmpi.wikispaces.com/file/view/sergiievskyi_et_al.pdf/513575848/sergiievskyi_et_al.pdf
Supporting information available online at:
http://compchemmpi.wikispaces.com/file/view/SuppInf_sergiievskyi_et_al_07-04-2014.pdf/513576008/SuppInf_sergiievskyi_et_al_07-04-2014.pd
Hydration of Clays at the Molecular Scale: The Promising Perspective of Classical Density Functional Theory
We report here how the hydration of complex surfaces can be efficiently
studied thanks to recent advances in classical molecular density functional
theory. This is illustrated on the example of the pyrophylite clay. After
presenting the most recent advances, we show that the strength of this implicit
method is that (i) it is in quantitative or semi-quantitative agreement with
reference all-atoms simulations (molecular dynamics here) for both the
solvation structure and energetics, and that (ii) the computational cost is two
to three orders of magnitude less than in explicit methods. The method remains
imperfect, in that it locally overestimates the polarization of water close to
hydrophylic sites of the clay. The high numerical efficiency of the method is
illustrated and exploited to carry a systematic study of the electrostatic and
van der Waals components of the surface-solvant interactions within the most
popular force field for clays, CLAYFF. Hydration structure and energetics are
found to weakly depend upon the electrostatics. We conclude on the consequences
of such findings in future force-field development.Comment: 24 pages, 8 figures. Molecular Physics (2014
Electronic Excited State Lifetimes of Anionic Water Clusters: Dependence on Charge Solvation Motif
An ongoing controversy about water cluster anions concerns the electron-binding motif, whether the charge center is localized at the surface or within the cluster interior. Here, mixed quantum-classical dynamics simulations have been carried out for a wide range of cluster sizes (n ≤ 1000) for [(H2O)n]- and [(D2O)n]- , based on a non-equilibrium first-order rate constant approach. The computed data are in good general agreement with time-resolved photoelectron imaging results (n ≤ 200). The analysis reveals that, for surface state electrons, the cluster size dependence of the excited state electronic energy gap and the magnitude of the non-adiabatic couplings have compensating influences on the excited state lifetimes: the excited state lifetime for surface states reaches a minimum for n ~ 150 and then increases for larger clusters. It is concluded that the electron resides in a surface-localized motif in all of these measured clusters, dominating at least up to n = 200
Molecular Density Functional Theory for water with liquid-gas coexistence and correct pressure
The solvation of hydrophobic solutes in water is special because liquid and
gas are almost at coexistence. In the common hypernetted chain approximation to
integral equations, or equivalently in the homogenous reference fluid of
molecular density functional theory, coexistence is not taken into account.
Hydration structures and energies of nanometer-scale hydrophobic solutes are
thus incorrect. In this article, we propose a bridge functional that corrects
this thermodynamic inconsistency by introducing a metastable gas phase for the
homogeneous solvent. We show how this can be done by a third order expansion of
the functional around the bulk liquid density that imposes the right pressure
and the correct second order derivatives. Although this theory is not limited
to water, we apply it to study hydrophobic solvation in water at room
temperature and pressure and compare the results to all-atom simulations. With
this correction, molecular density functional theory gives, at a modest
computational cost, quantitative hydration free energies and structures of
small molecular solutes like n-alkanes, and of hard sphere solutes whose radii
range from angstroms to nanometers. The macroscopic liquid-gas surface tension
predicted by the theory is comparable to experiments. This theory gives an
alternative to the empirical hard sphere bridge correction used so far by
several authors.Comment: 18 pages, 6 figure
Molecular Density Functional Theory of Water describing Hydrophobicity at Short and Long Length Scales
We present an extension of our recently introduced molecular density
functional theory of water [G. Jeanmairet et al., J. Phys. Chem. Lett. 4, 619,
2013] to the solvation of hydrophobic solutes of various sizes, going from
angstroms to nanometers. The theory is based on the quadratic expansion of the
excess free energy in terms of two classical density fields, the particle
density and the multipolar polarization density. Its implementation requires as
input a molecular model of water and three measurable bulk properties, namely
the structure factor and the k-dependent longitudinal and transverse dielectric
susceptibilities. The fine three-dimensional water structure around small
hydrophobic molecules is found to be well reproduced. In contrast the computed
solvation free-energies appear overestimated and do not exhibit the correct
qualitative behavior when the hydrophobic solute is grown in size. These
shortcomings are corrected, in the spirit of the Lum-Chandler-Weeks theory, by
complementing the functional with a truncated hard-sphere functional acting
beyond quadratic order in density. It makes the resulting functional compatible
with the Van-der-Waals theory of liquid-vapor coexistence at long range.
Compared to available molecular simulations, the approach yields reasonable
solvation structure and free energy of hard or soft spheres of increasing size,
with a correct qualitative transition from a volume-driven to a surface-driven
regime at the nanometer scale.Comment: 24 pages, 8 figure
Quantized time correlation function approach to non-adiabatic decay rates in condensed phase: Application to solvated electrons in water and methanol
A new, alternative form of the golden rule formula defining the non-adiabatic transition rate between two quantum states in condensed phase is presented. The formula involves the quantum time correlation function of the energy gap, of the non-adiabatic coupling, and their cross terms. Those quantities can be inferred from their classical counterparts, determined via MD simulations. The formalism is applied to the problem of the non-adiabatic relaxation of an equilibrated p-electron in water and methanol. We find that, in both solvent, the relaxation is induced by the coupling to the vibrational modes and the quantum effects modify the rate by a factor of 2-10 depending on the quantization procedure applied. The resulting p-state lifetime for a hypothetical equilibrium excited state appears extremely short, in the sub-100 fs regime. Although this result is in contrast with all previous theoretical predictions, we also illustrate that the lifetimes computed here are very sensitive to the simulated electronic quantum gap and to the strongly correlated non-adiabatic coupling
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