Molecular Dynamics Simulations of a Silver Atom in Water: Evidence for a Dipolar Excitonic State Riccardo Spezia, * Ce ´dric Nicolas, and Anne Boutin † Laboratoire de Chimie Physique, UMR 8000, CNRS, Universite ´ de Paris-Sud, 91405 Orsay Cedex, France Rodolphe Vuilleumier ‡ Laboratoire de Physique The ´orique des Liquides, UMR 7600, CNRS, Universite ´ Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France (Received 23 May 2003; published 13 November 2003) The properties of a silver atom in bulk water were studied for the first time by molecular dynamics simulations using two complementary mixed quantum-classical approaches. The first one consists of treating by quantum mechanics one electron only, which interacts with a classical silver cation and solvent through one-electron pseudopotentials. The second one is Car-Parrinello molecular dynamics that treats all the valence electrons quantum-mechanically. Very good agreement is obtained between these two methods, and the calculated absorption spectrum of the solvated silver atom agrees very well with experimental data. Both simulations reveal that the silver atom is in the critical region for the appearance of a dipolar excitonic state and exhibits a dipole moment of 2D with large fluctuations of 1D. The structure of the solvation shell is also analyzed. DOI: 10.1103/PhysRevLett.91.208304 PACS numbers: 82.30.Fi, 34.70.+e, 61.20.Ja, 71.55.Jv Understanding elementary chemical events in terms of basic physical properties is a subject of considerable interest today. Among those, the electron transfer process has been widely studied, due to the importance of redox reactions in everyday physical chemistry and biophysics. Even in the simplest case of metal atoms in polar sol- vents, the optical and electrical properties of the solvated atom, which strongly depends on the metal-solvent sys- tem, are not fully understood. In some cases, such as sodium in water, the ionization is complete and leads to two well separated solvated species, i.e., the electron and the ion. In contrast, the reduction of silver cations in a radiolysis experiment or when solvated silver halide is exposed to light leads to the formation of silver atoms, followed by a multistep coalescence into charged metal- lic clusters [1]. The role of the solvent is of critical importance, as illustrated by the dependence on the po- larity of the solvent of the optical properties of solvated metal atoms [2]. The solvent indeed mediates effective interactions between the ion and the electron. These interactions depend on both solvent and ion, or atom properties. The precise nature of the effective electron- ion interactions can be usefully addressed by mixed quantum-classical simulations. We present here a comple- ment to earlier research focusing on alkali metals in solution [3]. The silver atom in water is studied for the first time, using two different simulation methods. Quantum classical molecular dynamics. — We per- formed a mixed quantum-classical molecular dynamics (QCMD) of an excess electron plus a silver cation in bulk water. Only the excess electron was treated quantum mechanically, using the Born-Oppenheimer approxima- tion. In addition to the Hellmann-Feynman forces acting on each classical degree of freedom due to the excess charge are those arising from the empirical models for the solvent and the cation. Water/water interactions were described by the SPC model [4], while for the excess electron/water interaction we used a pseudopotential de- veloped by Turi and Borgis [5]. Details on the method can be found in Ref. [6]. The Ag  =water molecule interactions were modeled by a point charge on silver q Ag  1e and a 6–12 Lennard Jones (LJ) term, where  Ag  =O  0:40 kJ=mol and  Ag  =O  2:78  A. These values were calibrated using classical molecular dynamics simula- tions of Ag  in 300 SPC water molecules to reproduce experimental structural and thermodynamical properties. Using these parameters, we obtained the maximum of silver oxygen radial distribution function (RDF) at 2.36 A ˚ and a coordination number of 5.5 water molecules, while the experimental values are 2.35 A ˚ and 4.5, respec- tively [7]. The calculated hydration enthalpy was about 120 kcal=mol, while the experimental value is around 127 kcal=mol. The discrepancy found for the coordina- tion number is inherent to our choice to limit ourselves to a very simple LJ two-body potential for the Ag  =water interaction [8,9]. It has been shown, for example, that the inclusion of dipolar and quadrupolar polarizability of Ag  greatly enhances the description of silver chloride [10]. A repulsive semilocal pseudopotential was also added to the Coulomb attraction in the electron Hamil- tonian to describe the electron=Ag  interaction. The co- efficients were evaluated in vacuum to reproduce the experimental electron/silver properties [11]. The periodic simulation box used in the QCMD con- tains 300 SPC water molecules, one Ag  cation, and an excess electron. A short simulation with 800 SPC was PHYSICAL REVIEW LETTERS week ending 14 NOVEMBER 2003 VOLUME 91, NUMBER 20 208304-1 0031-9007= 03=91(20)=208304(4)$20.00  2003 The American Physical Society 208304-1