Electrode potential dependent desolvation and resolvation of germanium(100) in contact with aqueous perchlorate electrolytes.

The electrode potential dependence of the hydration layer on an n-Ge(100) surface was studied by a combination of in situ and operando electrochemical attenuated total reflection infrared (ATR-IR) spectroscopy and real space density functional theory (DFT) calculations. Constant-potential DFT calculations were coupled to a modified generalised Poisson-Boltzmann ion distribution model and applied within an ab initio molecular dynamics (AIMD) scheme. As a result, potential-dependent vibrational spectra of surface species and surface water were obtained, both experimentally and by simulations. The experimental spectra show increasing absorbance from the Ge-H stretching modes at negative potentials, which is associated with an increased negative difference absorbance of water-related OH modes. When the termination transition of germanium from OH to H termination occurs, the surface switches from hydrophilic to hydrophobic. This transition is fully reversible. During the switching, the interface water molecules are displaced from the surface forming a "hydrophobic gap". The gap thickness was experimentally estimated by a continuum electrodynamic model to be ≈2 Å. The calculations showed a shift in the centre of mass of the interface water by ≈0.9 Å due to the surface transformation. The resulting IR spectra of the interfacial water in contact with the hydrophobic Ge-H show an increased absorbance of free OH groups, and a decreased absorbance of strongly hydrogen bound water. Consequently, the surface transformation to a Ge-H terminated surface leads to a surface which is weakening the H-bond network of the interfacial water in contact.

Verlet-like algorithms for Car-Parrinello molecular dynamics with unequal electronic occupations.

The ab initio molecular dynamics simulations of metallic, charged, and electrochemical systems require, in principle, the inclusion of unequally occupied electronic states. In this contribution, the general approach to work with fixed but arbitrary occupations within the Car-Parrinello molecular dynamics scheme is revisited, focusing on the procedure which is required to maintain the orthonormality constraints in the commonly used position-Verlet integrator. Expressions to constrain also the orbital velocities, as it is demanded by a velocity-Verlet integrator, are then derived. The generalized unequal-occupation SHAKE algorithm is compared with the standard procedure for damped dynamics (energy optimization) of systems including fully unoccupied electronic states. In turn, the proposed unequal-occupation RATTLE algorithm is validated by the corresponding microcanonical ensemble simulations. It is shown that only with the proper orthogonalization method, a correct ordering of states and energy conserving dynamics can be achieved.

High Order Compact Multigrid Solver for Implicit Solvation Models.

The electrostatic problem defined by the continuum solvation models used in molecular mechanics and ab initio molecular dynamics is solved in real space through multiscale methods. First, the Poisson equation is rewritten as a stationary convection-diffusion equation and discretized by a general mesh size fourth-order compact difference scheme. Then, the linear system associated with such a discrete version of the elliptic partial differential equation is solved by a parallel (geometric) multigrid solver whose convergence rates and robustness are improved by an iterant recombination technique in which the multigrid acts as a preconditioner of a Krylov subspace method. The numerical tests performed on ideal and physical systems described by linear Poisson equations under different boundary conditions show the good performance of this accelerated multigrid solver. Furthermore, nonlinear Poisson equations, like the regular modified Poisson-Boltzmann equation, can also be solved by using in addition simple iterative schemes.