CP2K is an open source electronic structure and molecular dynamics software package to perform atomistic simulations of solid-state, liquid, molecular, and biological systems. It is especially aimed at massively parallel and linear-scaling electronic structure methods and state-of-the-art ab initio molecular dynamics simulations. Excellent performance for electronic structure calculations is achieved using novel algorithms implemented for modern high-performance computing systems. This review revisits the main capabilities of CP2K to perform efficient and accurate electronic structure simulations. The emphasis is put on density functional theory and multiple post–Hartree–Fock methods using the Gaussian and plane wave approach and its augmented all-electron extension.

The diffusion of protons through water is understood within the framework of the Grotthuss mechanism, which requires that they undergo structural diffusion in a stepwise manner throughout the water network. Despite long study, this picture oversimplifies and neglects the complexity of the supramolecular structure of water. We use first-principles simulations and demonstrate that the currently accepted picture of proton diffusion is in need of revision. We show that proton and hydroxide diffusion occurs through periods of intense activity involving concerted proton hopping followed by periods of rest. The picture that emerges is that proton transfer is a multiscale and multidynamical process involving a broader distribution of pathways and timescales than currently assumed. To rationalize these phenomena, we look at the 3D water network as a distribution of closed directed rings, which reveals the presence of medium-range directional correlations in the liquid. One of the natural consequences of this feature is that both the hydronium and hydroxide ion are decorated with proton wires. These wires serve as conduits for long proton jumps over several hydrogen bonds.T he mechanism by which protons move through water is at the heart of acid-base chemistry reactions. Understanding the reaction coordinates of this process has been one of the most challenging problems in physical chemistry due to the sheer complexity of water's hydrogen bond network (1-4). Developing a molecular basis for these phenomena is of great relevance in energy conversion applications such as in the design of efficient fuel cells (5). Over 200 y ago, von Grotthuss proposed a mechanism by which water would undergo electrolytic decomposition (6). He imagined that proton conduction involved the collective shuttling of hydrogen atoms along water wires. The early 20th century found many of the great scientists of the time developing conceptual models to understand the properties of water and its constituent ions (7,8). Detailed insights into the mechanisms of proton transfer (PT) came much later from a combination of both ab initio molecular dynamics (AIMD) simulations (3, 9-13) and force-field approaches based on the empirical valence bond formalism (14-16). The current textbook picture of the Grotthuss mechanism that has resulted from these studies involves a stepwise hopping of the proton from one water molecule to the next (1,17,18). This process occurs on a timescale of 1-2 ps. For a successful transfer, the model requires solvent reorganization around the proton-receiving species to develop a coordination pattern like that of the species it will convert to, a process known as presolvation. In all of these characterizations of the Grotthuss mechanism, the role of the connectivity of the water network was not brought to the forefront (3, 19).Sometimes PT has also been thought to take on coherent character involving jumps of several protons simultaneously. In this spirit, Eigen (20) suggested that the proton could delocalize over extended hydrogen...

We present a new method which combines Car-Parrinello and Born-Oppenheimer molecular dynamics in order to accelerate density functional theory based ab initio simulations. Depending on the system a gain in efficiency of 1 to 2 orders of magnitude has been observed, which allows ab initio molecular dynamics of much larger time and length scales than previously thought feasible. It will be demonstrated that the dynamics is correctly reproduced and that high accuracy can be maintained throughout for systems ranging from insulators to semiconductors and even to metals in condensed phases. This development considerably extends the scope of ab initio simulations.

We present an overview of recent static and time-resolved vibrational spectroscopic studies of liquid water from ambient conditions to the supercooled state, as well as of crystalline and amorphous ice forms. The structure and dynamics of the complex hydrogen-bond network formed by water molecules in the bulk and interphases are discussed, as well as the dissipation mechanism of vibrational energy throughout this network. A broad range of water investigations are addressed, from conventional infrared and Raman spectroscopy to femtosecond pump-probe, photon-echo, optical Kerr effect, sum-frequency generation, and two-dimensional infrared spectroscopic studies. Additionally, we discuss novel approaches, such as two-dimensional sum-frequency generation, three-dimensional infrared, and two-dimensional Raman terahertz spectroscopy. By comparison of the complementary aspects probed by various linear and nonlinear spectroscopic techniques, a coherent picture of water dynamics and energetics emerges. Furthermore, we outline future perspectives of vibrational spectroscopy for water researches.

Graphite and diamond have comparable free energies, yet forming diamond from graphite is far from easy. In the absence of a catalyst, pressures that are significantly higher than the equilibrium coexistence pressures are required to induce the graphite-to-diamond transition [1][2][3][4][5][6][7] . Furthermore, the formation of the metastable hexagonal polymorph of diamond instead of the more stable cubic diamond is favored at lower temperatures 2,5-7 . The concerted mechanism suggested in previous theoretical studies 8-12 cannot explain these phenomena. Using an ab initio quality neural-network potential 13 we performed a largescale study of the graphite-to-diamond transition assuming that it occurs via nucleation. The nucleation mechanism accounts for the observed phenomenology and reveals its microscopic origins. We demonstrated that the large lattice distortions that accompany the formation of the diamond nuclei inhibit the phase transition at low pressure and direct it towards the hexagonal diamond phase at higher pressure. The nucleation mechanism proposed in this work is an important step towards a better understanding of structural transformations in a wide range of complex systems such as amorphous carbon and carbon nanomaterials.Static compression of hexagonal graphite (HG) results in the formation of metastable hexagonal diamond (HD) at temperatures around 1200-1700 K 2,5-7 and cubic diamond (CD) at higher temperatures 1,[3][4][5]7 . Although the transition pressure is sensitive to the nature of the graphite samples neither of the diamond phases has been observed to form below ∼12 GPa. This pressure is significantly higher than the graphite-diamond coexistence pressure approximated by the Berman-Simon line P (GPa) ∼ 0.76 + 2.78 × 10 −3 T (K) 14 .Despite being an area of intense theoretical research 8-12 the microscopic mechanism of the formation of metastable HD and the reason for the remarkable stability of graphite above the coexistence pressure are still unknown. Computer simulations, which could help resolve these issues, have been hindered because of the inability of empirical potentials to describe the energetics of the transformation accurately 13,15 and the computational expense of more reliable ab initio methods. In the latter case, short simulation time and small system size (i.e. several hundred atoms) force the transition to occur in a concerted manner with the ultrafast (∼ 10 −2 − 1 ps) synchronous formation of all new chemical bonds accross the entire simulation box 11,12,16 . While concerted mechanisms can be observed at shock compression 16-18 , the transformation under static conditions is expected to proceed via nucleation and growth.It has been estimated that because diamond has an extremely high surface energy 19 its critical nuclei may contain thousands of atoms 20-22 . Hence, tens or even hundreds of thousands of atoms are required for modeling the diamond nuclei and the surrounding graphite matrix. Direct ab initio simulations of systems of this size are outright impossible. Ther...

Progress in the atomic-scale modelling of matter over the past decade has been tremendous. This progress has been brought about by improvements in methods for evaluating interatomic forces that work by either solving the electronic structure problem explicitly, or by computing accurate approximations of the solution and by the development of techniques that use the Born-Oppenheimer (BO) forces to move the atoms on the BO potential energy surface. As a consequence of these developments it is now possible to identify stable or metastable states, to sample configurations consistent with the appropriate thermodynamic ensemble, and to estimate the kinetics of reactions and phase transitions. All too often, however, progress is slowed down by the bottleneck associated with implementing new optimization algorithms and/or sampling techniques into the many existing electronic-structure and empirical-potential codes. To address this problem, we are thus releasing a new version of the i-PI software. This piece of software is an easily extensible framework for implementing advanced atomistic simulation techniques using interatomic potentials and forces calculated by an external driver code. While the original version of the code[1] was developed with a focus on path integral molecular dynamics techniques, this second release of i-PI not only includes several new advanced path integral methods, but also offers other classes of algorithms. In other words, i-PI is moving towards becoming a universal force engine that is both modular and tightly coupled to the driver codes that evaluate the potential energy surface and its derivatives.

Interpretation of the X-ray spectra of water as evidence for its asymmetric structure has challenged the conventional symmetric nearly tetrahedral model and initiated an intense debate about the order and symmetry of the hydrogen-bond network in water. Here we present new insights into the nature of local interactions in water obtained using a novel energy-decomposition method. Our simulations reveal that although a water molecule forms, on average, two strong donor and two strong acceptor bonds, there is a significant asymmetry in the energy of these contacts. We demonstrate that this asymmetry is a result of small instantaneous distortions of hydrogen bonds, which appear as fluctuations on a time scale of hundreds of femtoseconds around the average symmetric structure. Furthermore, we show that the distinct features of the X-ray absorption spectra originate from molecules with high instantaneous asymmetry. Our findings have important implications as they help reconcile the symmetric and asymmetric views on the structure of water.

Using the recently developed Car-Parrinello-like approach to Born-Oppenheimer molecular dynamics ( Kühne, T. D. ; et al. Phys. Rev. Lett. 2007 , 98 , 066401. ), we assess the accuracy of ab initio molecular dynamics at the semilocal density functional level of theory to describe structural and dynamic properties of liquid water at ambient conditions. We have performed a series of large-scale simulations using a number of parameter-free exchange and correlation functionals, to minimize and investigate the influence of finite size effects as well as statistical errors. We find that finite size effects in structural properties are rather small and, given an extensive sampling, reproducible. On the other hand, the influence of finite size effects on dynamical properties are much larger than generally appreciated. So much so that the infinite size limit is practically out of reach. However, using a finite size scaling procedure, thanks to the greater effectiveness of our new method we can estimate both the thermodynamic value of the diffusion coefficient and the shear viscosity. The hydrogen bond network structure and its kinetics are consistent with the conventional view of tetrahedrally coordinated water.