Theoretical characterization of temperature and density dependence of liquid water electronic excitation energy: comparison with recent experimental data.

In a recent paper [Aschi et al., ChemPhysChem 6, 53 (2005)], we characterized, by means of theoretical-computational procedures, the electronic excitation of water along the typical liquid state isochore (55.32 mol/l) 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 (2006)], 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 method), 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, (1) for all the densities and temperatures investigated, our calculated absorption spectra are in very good agreement with the experimental ones and (2) 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.