On the nature of charge-transfer excitations for molecules in aqueous solution: a polarizable QM/MM study

We illustrate the effect of solvation on the nature of electronic excitations of organic molecules which possess excited states of charge-transfer character. The analysis is carried out using both a continuum model and a polarizable QM/MM method that treats the solvent atomistically and embeds each atom in the solvent with a fluctuating charge which responds to the solute quantum-mechanical electrostatic potential in a self-consistent manner. We also show how solvation dynamics can influence the nature of the excited state of molecular systems. The application of the model to aqueous solutions of doxorubicin and a substituted polythiophene derivative shows that the solvent significantly affects the nature the excited states, which results in an enhanced or reduced charge-transfer character as measured using two of the most popular indices for evaluating the distance traveled by the electrons upon excitation

Fine tuning of atomic point charges classical simulations of pyridine in different environments

Abstract A correct description of electrostatic contributions in force fields for classical simulations is mandatory for an accurate modeling of molecular interactions in pure liquids or solutions. Here, we propose a new protocol for point charge fitting, aimed to take into the proper account different polarization effects due to the environment employing virtual sites and tuning the point charge within the polarizable continuum model framework. The protocol has been validated by means of molecular dynamics simulations on pure pyridine liquid and on pyridine aqueous solution, reproducing a series of experimental observables and providing the information for their correct interpretation at atomic level

Adsorption Geometry of Alizarin on Silver Nanoparticles: A Computational and Spectroscopic Study

The knowledge of the adsorption geometry of an analyte on a metal substrate employed in surface enhanced Raman scattering (SERS) spectroscopy is important information for the correct interpretation of experimental data. The adsorption geometry of alizarin on silver nanoparticles was studied through ab initio calculations in the framework of density functional theory (DFT) by modeling alizarin taking into account all the different charged species present in solution as a function of pH. The calculations allowed a faithful reproduction of the measured SERS spectra and to elucidate the adsorption geometry of this dye on the silver substrate