Statistical average of model orbital potentials for extended systems: Calculation of the optical absorption spectrum of liquid water (original) (raw)
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Insights into the ultraviolet spectrum of liquid water from model calculations
Journal of Chemical …, 2010
With a view toward a better molecular level understanding of the effects of hydrogen bonding on the ultraviolet absorption spectrum of liquid water, benchmark electronic structure calculations using high level wave function based methods and systematically enlarged basis sets are reported for excitation energies and oscillator strengths of valence excited states in the equilibrium water monomer and dimer and in a selection of liquid-like dimer structures. Analysis of the electron density redistribution associated with the two lowest valence excitations of the water dimer shows that these are usually localized on one or the other monomer, although valence hole delocalization can occur for certain relative orientations of the water molecules. The lowest excited state is mostly associated with the hydrogen bond donor and the significantly higher energy second excited state mostly with the acceptor. The magnitude of the lowest excitation energies is strongly dependent on where the valence hole is created, and only to a lesser degree on the perturbation of the excited electron density distribution by the neighboring water molecule. These results suggest that the lowest excitation energies in clusters and liquid water can be associated with broken acceptor hydrogen bonds, which provide energetically favorable locations for the formation of a valence hole. Higher valence excited states of the dimer typically involve delocalization of the valence hole and/or delocalization of the excited electron and/or charge transfer. Two of the higher valence excited states that involve delocalized valence holes always have particularly large oscillator strengths. Due to the pervasive delocalization and charge transfer, it is suggested that most condensed phase water valence excitations intimately involve more than one water molecule and, as a consequence, will not be adequately described by models based on perturbation of free water monomer states. The benchmark calculations are further used to evaluate a series of representative semilocal, global hybrid, and range separated hybrid functionals used in efficient time-dependent density functional methods. It is shown that such an evaluation is only meaningful when comparison is made at or near the complete basis set limit of the wave function based reference method. A functional is found that quantitatively describes the two lowest excitations of water dimer and also provides a semiquantitative description of the higher energy valence excited states. This functional is recommended for use in further studies on the absorption spectrum of large water clusters and of condensed phase water.
Theoretical Characterisation of the Electronic Excitation in Liquid Water
ChemPhysChem, 2005
Because of its central role in basically all aspects of science, water is certainly one of the most extensively investigated substances, from a theoretical point of view. Many properties have been, in fact, theoretically addressed both in the isolated and condensed phases. Nevertheless, many aspects are still not completely understood and represent the focus of active theoretical interest. Among them, one of the most appealing is certainly the understanding of the electronic properties, in particular the photoabsorption features, in condensed phase. Liquid water experimentally shows, under ambient conditions, the 0-1 absorption maximum at 147 nm, that is, 88 kJ mol À1 shifted toward the blue with respect to the corresponding absorption in vacuum. This blue-shift is known to be more pronounced in ice than in liquid water, and it is also present in small water clusters. From these observations, it is well-established that such a blue-shift is to be mainly ascribed to the short contacts of the excited molecule with its solvation shell (the water dipole moment undergoes an inversion upon 0-1 excitation ). However, only a few theoretical studies have been so far devoted to modelling water photoabsorption in the condensed phase. The computational methods available nowadays are, in fact, able to provide extremely accurate information about the photoexcitation of isolated molecules. However, there are still many difficulties in modelling the same phenomenon in the condensed phase. The inclusion of electronic degrees of freedom (necessary for studying an electronic excitation) into a simulation of a large number of molecules (necessary for a reliable modelling of a condensed phase) is, in fact, still challenging from a computational point of view. In this context, we recently proposed a theoretical computational approach, the perturbed matrix method (PMM), whose main computational feature is the possibility of including, into a classical simulation algorithm, electronic degrees of freedom. In a number [a] M.
Ab-initio calculations of Many-Body effects in liquids: the electronic excitations of water
2006
We present ab-initio calculations of the excited state properties of liquid water in the framework of Many-Body Green's function formalism. Snapshots taken from molecular dynamics simulations are used as input geometries to calculate electronic and optical spectra, and the results are averaged over the different configurations. The optical absorption spectra with the inclusion of excitonic effects are calculated by solving the Bethe-Salpeter equation. These calculations are made possible by exploiting the insensitivity of screening effects to a particular configuration. The resulting spectra are strongly modified by many-body effects, both concerning peak energies and lineshapes, and are in good agreement with experiments.
The Journal of Chemical Physics, 2008
In a recent paper ͓Aschi et al., ChemPhysChem 6, 53 ͑2005͔͒, we characterized, by means of theoretical-computational procedures, the electronic excitation of water along the typical liquid state isochore ͑55.32 mol/ l͒ for a large range of temperature. In that paper we were able to accurately reproduce the experimental absorption maximum at room temperature and to provide a detailed description of the temperature dependence of the excitation spectrum along the isochore. In a recent experimental work by Marin et al. ͓J. Chem. Phys. 125, 104314 ͑2006͔͒, water electronic excitation energy was carefully analyzed in a broad range of density and temperature, finding a remarkable agreement of the temperature behavior of the experimental data with our theoretical results. Here, by means of the same theoretical-computational procedures ͑molecular dynamics simulations and the perturbed matrix method͒, we investigate water electronic absorption exactly in the same density-temperature range used in the experimental work, hence, now considering also the absorption density dependence. Our results point out that, ͑1͒ for all the densities and temperatures investigated, our calculated absorption spectra are in very good agreement with the experimental ones and ͑2͒ the gradual maxima redshift observed increasing the temperature or decreasing the density has to be ascribed to a real shift of the lowest X → A electronic transition, supporting the conclusions of Marin et al.
The Journal of Chemical Physics, 2009
The dynamic polarizability and optical absorption spectrum of liquid water in the 6-15 eV energy range are investigated by a sequential molecular dynamics ͑MD͒/quantum mechanical approach. The MD simulations are based on a polarizable model for liquid water. Calculation of electronic properties relies on time-dependent density functional and equation-of-motion coupled-cluster theories. Results for the dynamic polarizability, Cauchy moments, S͑−2͒, S͑−4͒, S͑−6͒, and dielectric properties of liquid water are reported. The theoretical predictions for the optical absorption spectrum of liquid water are in good agreement with experimental information.
The Journal of Chemical Physics, 2008
A quantum mechanics/molecular mechanics ͑QM/MM͒ type of scheme is employed to calculate the solvent-induced shifts of molecular electronic excitations. The effective fragment potential ͑EFP͒ method was used for the classical potential. Since EFP has a density dependent functional form, in contrast with most other MM potentials, time-dependent density functional theory ͑TDDFT͒ has been modified to combine TDDFT with EFP. This new method is then used to perform a hybrid QM/MM molecular dynamics simulation to generate a simulated spectrum of the n → ء vertical excitation energy of acetone in vacuum and with 100 water molecules. The calculated water solvent effect on the vertical excitation energy exhibits a blueshift of the n → ء vertical excitation energy in acetone ͑⌬ 1 = 0.211 eV͒, which is in good agreement with the experimental blueshift.
Brazilian journal of physics, 2004
Electronic properties of liquid water were investigated by sequential Monte Carlo/Quantum mechanics calculations. The density of states (DOS) and HOMO-LUMO gap (E-G) of liquid water have been determined by Hartree-Fock and Density Functional Theory (DFT) calculations. The quantum mechanical calculations were carried out over uncorrelated supermolecular structures generated by the Monte Carlo simulations. The DFT calculations were performed with a modified B3LYP exchange-correlation functional proposed by Abu-Awwad and Politzer which was parametrized to reproduce valence orbital energies in agreement with experimental ionization potentials of the water molecule. We have analyzed the dependence of the DOS and HOMO-LUMO gap on the number of water molecules and on surface effects. Our prediction for E-G is 6.5 +/- 10.5 eV in good agreement with a recent experimental prediction of 6.9 eV.
Chemical Physics, 1995
For the calculation of stationary molecular spectra it is a well established technique to propagate wave packets on related potential energy surfaces. We show that the time-dependent formulation of the absorption coefficient can be generalized to a case where the molecular system interacts with a dissipative environment. Therefore, we utilize the concept of the time-dependent statistical operator reduced to some selected molecular degrees of freedom involved in the optical transition. The related quantum master equation defines the dissipative time propagation which is used, after an appropriate Fourier transformation, to derive the absorption coefficient. The environmental degrees of freedom enter the absorption via the various elements of the so-called Redfield tensor. The method is used to calculate the infrared absorption within a single potential energy surface of the O-H stretching vibration of the water molecule. As a second example, the UV-absorption is discussed resulting from an electronic transition into the coupled valence and Rydberg states of matrix isolated NO. 0301-0104/95/$09.50 (~) 1995 Elsevier Science B.V. All rights reserved SSDI 0301-01 04(95)00326-6
Physical chemistry chemical physics : PCCP, 2015
Knowledge about the intrinsic electronic properties of water is imperative for understanding the behaviour of aqueous solutions that are used throughout biology, chemistry, physics, and industry. The calculation of the electronic band gap of liquids is challenging, because the most accurate ab initio approaches can be applied only to small numbers of atoms, while large numbers of atoms are required for having configurations that are representative of a liquid. Here we show that a high-accuracy value for the electronic band gap of water can be obtained by combining beyond-DFT methods and statistical time-averaging. Liquid water is simulated at 300 K using a plane-wave density functional theory molecular dynamics (PW-DFT-MD) simulation and a van der Waals density functional (optB88-vdW). After applying a self-consistent GW correction the band gap of liquid water at 300 K is calculated as 7.3 eV, in good agreement with recent experimental observations in the literature (6.9 eV). For si...
Molecular Density Functional Theory of Water
The Journal of Physical Chemistry Letters, 2013
Three-dimensional implementations of liquid-state theories offer an efficient alternative to computer simulations for the atomic-level description of aqueous solutions in complex environments. In this context, we present a (classical) molecular density functional theory (MDFT) of water that is derived from first principles and is based on two classical density fields, a scalar one, the particle density, and a vectorial one, the multipolar polarization density. Its implementation requires as input the partial charge distribution of a water molecule and three measurable bulk properties, namely, the structure factor and the k-dependent longitudinal and transverse dielectric constants. It has to be complemented by a solute−solvent threebody term that reinforces tetrahedral order at short-range. The approach is shown to provide the correct 3-D microscopic solvation profile around various molecular solutes, possibly possessing H-bonding sites, at a computer cost two to three orders of magnitude lower than with explicit simulations.