| Abstract
| - The absorption spectra of aminocoumarin C151 in water and n-hexane solution are investigated by an explicitquantum chemical solvent model. We improved the efficiency of the frozen-density embedding scheme, asused in a former study on solvatochromism (J. Chem. Phys. 2005, 122, 094115) to describe very large solventshells. The computer time used in this new implementation scales approximately linearly (with a low prefactor)with the number of solvent molecules. We test the ability of the frozen-density embedding to describe specificsolvent effects due to hydrogen bonding for a small example system, as well as the convergence of theexcitation energy with the number of solvent molecules considered in the solvation shell. Calculations withup to 500 water molecules (1500 atoms) in the solvent system are carried out. The absorption spectra arestudied for C151 in aqueous or n-hexane solution for direct comparison with experimental data. To obtainsnapshots of the dye molecule in solution, for which subsequent excitation energies are calculated, we use aclassical molecular dynamics (MD) simulation with a force field adapted to first-principles calculations. Inthe calculation of solvatochromic shifts between solvents of different polarity, the vertical excitation energyobtained at the equilibrium structure of the isolated chromophore is sometimes taken as a guess for the excitationenergy in a nonpolar solvent. Our results show that this is, in general, not an appropriate assumption. This ismainly due to the fact that the solute dynamics is neglected. The experimental shift between n-hexane andwater as solvents is qualitatively reproduced, even by the simplest embedding approximation, and the resultscan be improved by a partial polarization of the frozen density. It is shown that the shift is mainly due to theelectronic effect of the water molecules, and the structural effects are similar in n-hexane and water. Byincluding water molecules, which might be directly involved in the excitation, in the embedded region, anagreement with experimental values within 0.05 eV is achieved.
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