A path-integral Car-Parrinello molecular dynamics simulation of liquid water and ice is performed. It is found that the inclusion of nuclear quantum effects systematically improves the agreement of first-principles simulations of liquid water with experiment. In addition, the proton momentum distribution is computed utilizing a recently developed open path-integral molecular dynamics methodology. It is shown that these results are in good agreement with experimental data.

The transport of protons through aqueous, partially aqueous, or nonaqueous hydrogen-bonded media is a fundamental process in many biologically and technologically important systems. Liquid methanol is an example of a hydrogen-bonded system that, like water, supports anomalously fast proton transport. Using the methodology of ab initio molecular dynamics, in which internuclear forces are computed directly from electronic structure calculations as the simulation proceeds, we have investigated the microscopic mechanism of the proton transport process in liquid methanol at 300 K. It is found that the defect structure associated with an excess proton in liquid methanol is a hydrogen-bonded cationic chain whose length generally exceeds the average chain length in pure liquid methanol. Hydrogen bonds in the first and second solvation shells of the excess proton are considerably shorter and stronger than ordinary methanol–methanol hydrogen bonds. Along this chain, proton transfer reactions occur in an essentially random manner described by Poisson statistics. Structural diffusion of the defect structure is possible if the proton migrates toward an end of the defect chain, which causes a weakening of the hydrogen bonds at the opposite end. The latter can, therefore, be easily ruptured by ordinary thermal fluctuations. At the end of the chain where the proton resides, new hydrogen bonds are likely to form due to the strong associative nature of the excess proton. It is through this “snake-like” mechanism that the defect structure is able to diffuse through the hydrogen-bond network of the liquid. The estimated activation enthalpy of this proposed mechanism is found to be in reasonable agreement with the experimentally determined activation enthalpy.

Free-energy-based simulations are increasingly providing the narratives about the structures, dynamics and biological mechanisms that constitute the fabric of protein science. Here, we review two recent successes. It is becoming practical: (i) to fold small proteins with free-energy methods without knowing substructures, and (ii) to compute ligand-protein binding affinities, not just their binding poses. Over the past 40 years, the timescales that can be simulated by atomistic MD are doubling every 1.3 years – which is faster than Moore's law. Thus, these advances are not simply due to the availability of faster computers. Force fields, solvation models and simulation methodology have kept pace with computing advancements, and are now quite good. At the tip of the spear recently are GPU-based computing, improved fast-solvation methods, continued advances in force fields, and conformational sampling methods that harness external information.

Glasses are dynamically arrested states of matter that do not exhibit the long-range periodic structure of crystals 1-4 . Here we develop new insights from theory and simulation into the impact of quantum fluctuations on glass formation. As intuition may suggest, we observe that large quantum fluctuations serve to inhibit glass formation as tunnelling and zero-point energy allow particles to traverse barriers facilitating movement. However, as the classical limit is approached a regime is observed in which quantum effects slow down relaxation making the quantum system more glassy than the classical system. This dynamical 'reentrance' occurs in the absence of obvious structural changes and has no counterpart in the phenomenology of classical glass-forming systems.Although a wide variety of glassy systems ranging from metallic to colloidal can be accurately described using classical theory, quantum systems ranging from molecular, to electronic and magnetic form glassy states 5,6 . Perhaps the most intriguing of these is the coexistence of superfluidity and dynamical arrest, namely the 'superglass' state suggested by recent numerical, theoretical and experimental work [7][8][9] . Although such intriguing examples exist, at present there is no unifying framework to treat the interplay between quantum and glassy fluctuations in the liquid state.To attempt to formulate a theory for a quantum liquid to glass transition, we may first appeal to the classical case for guidance. Here, a microscopic theory exists in the form of mode-coupling theory (MCT), which requires only simple static structural information as input and produces a full range of dynamical predictions for time correlation functions associated with single-particle and collective fluctuations 10 . Although MCT has a propensity to overestimate a liquid's tendency to form a glass, it has been shown to account for the emergence of the non-trivial growing dynamical length scales associated with vitrification 11 . Perhaps more importantly, MCT has made numerous non-trivial predictions ranging from logarithmic temporal decay of density fluctuations and reentrant dynamics in adhesive colloidal systems to various predictions concerning the effect of compositional mixing on glassy behaviour 12,13 . These have been confirmed by both simulation and experiment [14][15][16] .A fully microscopic quantum version of MCT (QMCT) that requires only the observable static structure factor as input may be developed along the same lines as the classical version. Indeed, a zero-temperature version of such a theory has been developed and successfully describes the wave-vector-dependent dispersion in superfluid helium 17 . In the Supplementary Information, we outline the derivation of a full temperature-dependent QMCT. In the limit of high temperatures, our theory reduces to the hard-sphere fluid. φ is the volume fraction, Λ * = (βh 2 /mσ 2 ) 1/2 is the thermal wavelength in units of inter-particle separation σ , and β = 1/k B T is the inverse temperature. The approach by which the...

Ab initio molecular dynamics simulations are employed to study the structural and proton transport properties of methanol-water mixtures. Structural characteristics analyzed at two different methanol mole fractions (X(M) = 0.25 and X(M) = 0.5) reveal enhanced structuring of water as the methanol mole fraction increases in agreement with recent neutron diffraction experiments. The simulations reveal the existence of separate hydrogen-bonded water and methanol networks, also in agreement with the neutron diffraction data. The addition of a single proton to the X(M) = 0.5 mixture leads to an anomalous structural or Grotthuss-type diffusion mechanism of the charge defect in which water-to-water, methanol-to-water, and water-to-methanol proton transfer reactions play the dominant role with methanol-to-methanol transfers being much less significant. Unlike in bulk water, where coordination number fluctuations drive the proton transport process, suppression of the coordination number of waters in the first solvation shell of the defect diminish the importance of coordination number fluctuations as a driving force in the structural diffusion process. The charge defect is found to reside preferentially at the interface between water and methanol networks. The length of the ab initio molecular dynamics run (approximately 120 ps), allowed the diffusion constant of the charge defect to be computed, yielding a value of D = 4.2 x 10(-5) cm2/s when deuterium masses are assigned to all protons in the system. The relation of this value to excess proton diffusion in bulk water is discussed. Finally, a kinetic theory is introduced to identify the relevant time scales in the proton transfer/transport process.