Optical properties of pyridine and methyl-pyridinium in water using DFT/MM

Molecular properties such as excitation energies, polarisabilities and two-photon absorption cross-sections of pyridine and N-methyl-pyridinium embedded in a solvent of water are studied using density functional theory/molecular mechanics (DFT/MM) calculations. The purpose of this study is to study how well the differences between a cationic and a neutral specie can be predicted with water as the solvent. In order to answer this question, the DFT/MM method used is benchmarked in terms of convergence profiles, e.g. how many calculations are needed in order to obtain a statistically converged result. Quantification of for example how many water molecules needs to be included in the quantum mechanics system in the calculations before a converged molecular property is obtained which is also benchmarked. The latter will serve as valuable information in further calculations of larger organic molecules, where more experimental data are available. In this work, the calculated excitation energies are compared to experimentally determined ultraviolet/visible (UV/VIS) spectra of both pyridine and N-methyl-pyridinium and a reasonable agreement is found.