O potential energy function*

Most commonly used interfaces between quantum mechanics and molecular mechanics (QM/MM) are based on the so-called electrostatic embedding scheme. There, an additional external potential term is included in the electronic structure calculation of the QM subsystem [1]. In this way the QM subsystem is polarized, whereas the charges of the MM subsystem remain static. This limits the applicability of the interface to the parameter set of the MM potential. For example, water potential functions based on fixed point charges are usually parameterized to reproduce a few thermally averaged properties of the liquid and may not be transferable to other systems such as at solid/liquid interfaces. The properties of the water molecule are strongly environment dependent as illustrated by the molecular dipole moment, which is 1.8 D in the gas phase but 3.1 D in ice Ih [2]. Polarizability must be taken into account in order to ensure transferability. An energy functional for describing mutual polarization between the QM and MM parts of the system is presented. A double SCF loop is introduced in order to arrive at a self-consistent solution for the total system. The resulting polarizable embedding QM/MM (PE-QM/MM) interface [3,4] couples KS-DFT with the single center multipole expansion (SCME) model describing H2O molecules. The SCME includes static dipole up to the hexadecapole moment tensor, as well as polarizable dipole and quadrupole moment tensors, resulting in a good description of small clusters, liquid and ice Ih [5,6]. A flexible boundary scheme has also been developed, a scattering adapted implementation of the FIRES approach (SAFIRES) [7]. Applications to dynamics simulations at solid/liquid interfaces will be presented, and a modified Poisson-Boltzmann distribution to describe the electric double layer at solid/ liquid interfaces in PE-QM/MM simulations will be discussed.