Towards a polarizable force field for molecular liquids

Modern time resolved spectroscopic techniques, such as off resonant transient birifrangence, or in the frequency domain, such as Fourier transform infrared, constitute a valuable experimental source of information for the dynamical and structural properties of molecular liquids. Many important spectroscopic physical observables can be in principle straightforwardly computed using classical molecular dynamics (MD) simulations. However, MD in its standard implementation has important drawbacks which severely limit its use in the study of light scattering phenomena. In particular, the lack of an adequate representation of the molecular electronic response to the external and local field prevents the accurate simulation of spectroscopic properties. The chemical potential equalization (CPE) method can be used to derive a realistic parameterization for describing electronic polarization effects. The CPE approach is a semi-empirical density functional method. The total molecular energy is expanded to the second order in terms of local atomic electron densities, represented by fluctuating point charges. The empirical parameters of the expansion are atomic electronegativity and hardness which can be fitted onto accurate ab initio calculations of the isolated molecule. CPE is parameterized so as to reproduce global ground state properties of the molecule such as polarizability, ionization potential and electronic affinity as well as local properties such as atomic charges and Fukui indices. From a computational standpoint, the CPE method is simple and inexpensive and can be either straightforwardly implemented in a Car-Parrinello fashion, or used as an analytic tool for computing, a posteriori, spectroscopic properties from trajectories generated by conventional MD simulations, Here we present an extension of the CPE method including atomic dipolar charge distributions and apply it to the simple case of the water monomer and dimer. Very encouraging results are found.