Electronic excitation of Cl- in liquid water and at the surface of a cluster: a sequential born - oppenheimer molecular dynamics/quantum mechanics approach

Abstract

A sequential molecular dynamics/quantum mechanics approach is applied to investigate the electronic excitation of Cl− in liquid water and in a water cluster. Time-dependent density functional theory (TDDFT) and equation-of-motion coupled-cluster with single and double excitations (EOM-CCSD) are used to calculate the excitation energies from Born−Oppenheimer molecular dynamics configurations. The selected configurations include a quantum system with the Cl− anion and a number of explicit water molecules (nw) as well as an embedding background defined by fractional point charges on the remaining water molecules. Our results indicate that for both the liquid and the cluster environments the excited electron is delocalized on the hydrogen atoms of the first hydration shell and in a nearby cavity. Convergence of the charge-transfer-to-solvent (CTTS) energy with the number of water molecules is observed for a quantum system embedded in the polarizing charge background for nw ≥ 3. Furthermore, we find that the CTTS energy of Cl− in both solution and cluster environments is very similar. The predicted CTTS energy threshold for the ionic solution (∼6.6 ± 0.3 eV) is in good agreement with experiment (6.8 and 7.1 eV).