An Explicit Quantum Chemical Method for Modeling Large Solvation Shells Applied to Aminocoumarin C151

Abstract

The absorption spectra of aminocoumarin C151 in water and n-hexane solution are investigated by an explicit quantum chemical solvent model. We improved the efficiency of the frozen-density embedding scheme, as used in a former study on solvatochromism (J. Chem. Phys. 2005, 122, 094115) to describe very large solvent shells. 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 specific solvent effects due to hydrogen bonding for a small example system, as well as the convergence of the excitation energy with the number of solvent molecules considered in the solvation shell. Calculations with up to 500 water molecules (1500 atoms) in the solvent system are carried out. The absorption spectra are studied for C151 in aqueous or n-hexane solution for direct comparison with experimental data. To obtain snapshots of the dye molecule in solution, for which subsequent excitation energies are calculated, we use a classical molecular dynamics (MD) simulation with a force field adapted to first-principles calculations. In the calculation of solvatochromic shifts between solvents of different polarity, the vertical excitation energy obtained at the equilibrium structure of the isolated chromophore is sometimes taken as a guess for the excitation energy in a nonpolar solvent. Our results show that this is, in general, not an appropriate assumption. This is mainly due to the fact that the solute dynamics is neglected. The experimental shift between n-hexane and water as solvents is qualitatively reproduced, even by the simplest embedding approximation, and the results can be improved by a partial polarization of the frozen density. It is shown that the shift is mainly due to the electronic effect of the water molecules, and the structural effects are similar in n-hexane and water. By including water molecules, which might be directly involved in the excitation, in the embedded region, an agreement with experimental values within 0.05 eV is achieved.

Equilibrium Geometries of Noncovalently Bound Intermolecular Complexes Derived from Subsystem Formulation of Density Functional Theory

“…For dipole-bound complexes, the errors in the binding energy are larger. The maximal relative overestimation of the binding energy for HCN-CH 3 SH reaches 30%, whereas the binding energy in CH 3 ClHCl is underestimated by 19%. For van der Waals complexes, KSCED LDA does not perform uniformly.…”

PyADF — A scripting framework for multiscale quantum chemistry

“…The new results object is then used to initialize and run an adfnmrjob (lines [24][25][26]. Finally, the get_all_shieldings method of the NMR job's results object is used to extract the calculated shieldings, which can then be printed (lines [28][29][30][31].…”

“…An important multiscale application of the FDE scheme is the calculation of solvent effects on molecular properties (e.g., electronic excitation energies, 29,30 ESR hyperfine coupling constants, 53 or NMR shieldings 31 ). Such calculations employ the sequential molecular dynamics followed by quantum-mechanics calculations (S-MD/QM) strategy.…”

“…Typically, they involve hundreds of individual calculations, and the results of a subset of these calculations are needed as input for following steps. For instance, applications of the FDE scheme to calculate solvent effects on molecular properties [29][30][31] require the construction of an approximate solvent electron density, followed by an FDE calculation of the molecular property of interest for the embedded solute molecule. This needs to be done for a large number of snapshots taken from a molecular dynamics simulation.…”

Physical Chemistry at the University of Geneva

“…Absorption spectra of aminocoumarin c151 in water (black) and n-hexane (red) obtained form multi-level simulations using orbital-free embedding potential to couple the solvent with the chromophore. [46] 4 , a compound dense in uranium and nitrogen, which can be used as a molecular precursor to high-purity UN, a potential nuclear fuel of the future.…”