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A novel algorithm, based on a hybrid Gaussian and plane waves (GPW) approach, is developed for the canonical second order Moller-Plesset perturbation energy (MP2) of finite and extended systems. The key aspect of the method is that the electron repulsion integrals (ia vertical bar lambda sigma) are computed by direct integration between the products of Gaussian basis functions lambda sigma and the electrostatic potential arising from a given occupied virtual pair density ia. The electrostatic potential is obtained in a plane waves basis set after solving the Poisson equation in Fourier space. In particular, for condensed phase systems, this scheme is highly efficient. Furthermore, our implementation has low memory requirements and displays excellent parallel scalability up to 100 000 processes. In this way, canonical MP2 calculations for condensed phase systems containing hundreds of atoms or more than 5000 basis functions can be performed within minutes, while systems up to 1000 atoms and 10 000 basis functions remain feasible. Solid LiH has been employed as a benchmark to study basis set and system size convergence. Lattice constants and cohesive energies of various molecular crystals have been studied with MP2 and double-hybrid functionals.

Understanding the properties of hydrated electrons, which were first observed using pulse radiolysis of water in 1962, is crucial because they are key species in many radiation chemistry processes. Although time-resolved spectroscopic studies and molecular simulations have shown that an electron in water (prepared, for example, by water photoionization) relaxes quickly to a localized, cavity-like structure similar to 2.5 angstrom in radius, this picture has recently been questioned. In another experimental approach, negatively charged water clusters of increasing size were studied with photoelectron and IR spectroscopies. Although small clusters can bind an excess electron, their character is very different from bulk hydrated species. As data on electron binding in liquid water have become directly accessible experimentally, the cluster-to-bulk extrapolations have become a topic of lively debate. Quantum electronic structure calculations addressing experimental measurables have, until recently, been largely limited to small clusters; extended systems were approached mainly with pseudopotential calculations combining a classical description of water with a quantum mechanical treatment of the excess electron. In this Account, we discuss our investigations of electrons solvated in water by means of ab initio molecular dynamics simulations. This approach, applied to a model system of a negatively charged cluster of 32 water molecules, allows us to characterize structural, dynamical, and reactive aspects of the hydrated electron using all of the system's valence electrons. We show that under ambient conditions, the electron localizes into a cavity close to the surface of the liquid cluster. This cavity is, however, more flexible and accessible to water molecules than an analogous area around negatively charged ions. The dynamical process of electron attachment to a neutral water cluster is strongly temperature dependent. Under ambient conditions, the electron relaxes in

We have carried out ``first-principles'' Born-Oppenheimer molecular dynamics (BOMD) simulations of the phosphate ions H2PO4- and HPO42- in liquid water and have calculated their IR spectra by Fourier transform techniques from the trajectories. IR bands were assigned by a so-called ``generalized normal coordinate analysis''. The effects of including Hartree-Fock (HF) exchange into the density functional theory (DFT) computation of forces were studied by comparing results obtained with the well-known BP, BLYP, and B3LYP functionals. The neglect of dispersion in the functionals was empirically corrected. The inclusion of HF exchange turned out to yield dramatically improved and, thus, quite accurate descriptions of the IR spectra observed for H2PO4- and HPO42- in aqueous solution. An analysis of earlier computational results (Klahn, M. et al. J. Phys. Chem. A 2004, 108, 6186-6194) on these vibrational spectra, which had been obtained in a hybrid setting combining a BP description of the respective phosphate with a simple molecular mechanics (MM) model of its aqueous environment, revealed three different sources of error, (i) the BP force field of the phosphates is much too soft and would have required a substantial scaling of frequencies, (ii) the oversimplified water force field entailed incorrect solvation structures and, thus, qualitatively wrong patterns of solvatochromic band shifts, and (iii) quantitative frequency computations additionally required the inclusion of HF exchange. Thus, the results of the B3LYP BOMD simulations do not only characterize physical properties like the IR spectra or the solvation structures of the phosphate systems but also provide clues for the future design of simplified but nevertheless reasonably accurate DFT/MM methods applicable to phosphates.

Resolving the atomic structure of the surface of ice particles within clouds, over the temperature range encountered in the atmosphere and relevant to understanding heterogeneous catalysis on ice, remains an experimental challenge. By using first-principles calculations, we show that the surface of crystalline ice exhibits a remarkable variance in vacancy formation energies, akin to an amorphous material. We find vacancy formation energies as low as ∼0.1-0.2 eV, which leads to a higher than expected vacancy concentration. Because a vacancy's reactivity correlates with its formation energy, ice particles may be more reactive than previously thought. We also show that vacancies significantly reduce the formation energy of neighbouring vacancies, thus facilitating pitting and contributing to pre-melting and quasi-liquid layer formation. These surface properties arise from proton disorder and the relaxation of geometric constraints, which suggests that other frustrated materials may possess unusual surface characteristics. © 2011 Macmillan Publishers Limited. All rights reserved.

Marcus theory of electron transfer is the quintessential example of a successful theory in physical chemistry. In this paper, we describe the theoretical approach we have adopted to compute key parameters in Marcus theory. In our method, based on molecular dynamics simulations and density functional theory, the redox center and its environment are treated at the same level of theory. Such a detailed atomistic model describes specific solvent-solute interactions, such as hydrogen bonding, explicitly. The quantum chemical nature of our computations enables us to study the effect of chemical modifications of the redox centers and deals accurately with the electronic polarization of the environment. Based on results of previous work, we will illustrate that quantitative agreement with experiment can be obtained for differences in redox potentials and solvent reorganization energies for systems ranging from small organic compounds to proteins in solution.