Implicit solvation models are widely used in electronic structure simulations of catalytic reactions in condensed phases, such as solvents and aqueous environments, but also for reaction centers embedded in proteins and other biologically relevant environments. In complex systems these quantum-classical embedding schemes often present the only viable methodology due to their simplicity and advantages regarding computational cost. These approaches only model a solute region explicitly while the interaction with the gross part of the solvent is coarse-grained into a continuum dielectric response, instead of considering individual solvent molecules. This dielectric response described by the solvent static dielectric permittivity ε, often enters such models as a free parameter. However, simply using the solvent’s experimental bulk dielectric constant neglects the fact that the dielectric response of the solvent deviates, sometimes drastically and anisotropically, from its isotropic bulk value in vicinity of the solute. To gain insight into its spatial dependence from molecular dynamics (MD) simulations, ε can be related to thermal equilibrium fluctuations of the polarization at zero field. The Kirkwood-Fröhlich (KF) approach usually applied in the derivation of such formulas faces two major difficulties: First, correlated polarization fluctuations are long-range and subject to slow convergence, making KF expensive in simulations. Second, the difficulty of the electrostatic boundary value problem one needs to solve in a KF ansatz increases with the system's complexity. In this contribution, we present a possible alternative procedure relying on a direct comparison of instantaneous polarization and Maxwell field for neutral and spatially compact subdomains. In this way, we gain access to the spatially resolved full-rank static permittivity tensor while also avoiding an artificial, a posteriori introduction of boundary conditions for phase separation in interfacial systems. Results are discussed for water bulk, dichloro-ethane bulk and water-dichloroethane liquid-liquid interface (LLI) systems, where the LLI serves as a model for soft biological interfaces.

Direct inversion approach to local permittivity at liquid-liquid interfaces

2019 Fall National Meeting

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