A series of Car-Parrinello (CP) molecular dynamics simulations of water are presented, aimed at assessing the accuracy of density functional theory in describing the structural and dynamical properties of water at ambient conditions. We found negligible differences in structural properties obtained using the Perdew-Burke-Ernzerhof or the Becke-Lee-Yang-Parr exchange and correlation energy functionals; we also found that size effects, although not fully negligible when using 32 molecule cells, are rather small. In addition, we identified a wide range of values of the fictitious electronic mass (micro) entering the CP Lagrangian for which the electronic ground state is accurately described, yielding trajectories and average properties that are independent of the value chosen. However, care must be exercised not to carry out simulations outside this range, where structural properties may artificially depend on micro. In the case of an accurate description of the electronic ground state, and in the absence of proton quantum effects, we obtained an oxygen-oxygen correlation function that is overstructured compared to experiment, and a diffusion coefficient which is approximately ten times smaller.

Water confined at the nanoscale has been the focus of numerous experimental and theoretical investigations in recent years, yet there is no consensus on such basic properties as diffusion and the nature of hydrogen bonding (HB) under confinement. Unraveling these properties is important to understand fluid flow and transport at the nanoscale, and to shed light on the solvation of biomolecules. Here we report on a first principle, computational study focusing on water confined between prototypical nonpolar substrates, i.e., single-wall carbon nanotubes and graphene sheets, 1-2.5 nm apart. The results of our molecular dynamics simulations show the presence of a thin, interfacial liquid layer (approximately 5 A) whose microscopic structure and thickness are independent of the distance between confining layers. The properties of the HB network are very similar to those of the bulk outside the interfacial region, even in the case of strong confinement. Our findings indicate that the perturbation induced by the presence of confining media is extremely local in liquid water, and we propose that many of the effects attributed to novel phases under confinement are determined by subtle electronic structure rearrangements occurring at the interface with the confining medium.

A series of 20 ps ab initio molecular dynamics simulations of water at ambient density and temperatures ranging from 300 to 450 K are presented. Car-Parrinello (CP) and Born-Oppenheimer (BO) molecular dynamics techniques are compared for systems containing 54 and 64 water molecules. At 300 K, an excellent agreement is found between radial distribution functions (RDFs) obtained with BO and CP dynamics, provided an appropriately small value of the fictitious mass parameter is used in the CP simulation. However, we find that the diffusion coefficients computed from CP dynamics are approximately two times larger than those obtained with BO simulations for T>400 K, where statistically meaningful comparisons can be made. Overall, both BO and CP dynamics at 300 K yield overstructured RDFs and slow diffusion as compared to experiment. In order to understand these discrepancies, the effect of proton quantum motion is investigated with the use of empirical interaction potentials. We find that proton quantum effects may have a larger impact than previously thought on structure and diffusion of the liquid.

Using quantum simulation techniques based on either density functional theory or quantum Monte Carlo, we find clear evidence of a first-order transition in liquid hydrogen, between a low conductivity molecular state and a high conductivity atomic state. Using the temperature dependence of the discontinuity in the electronic conductivity, we estimate the critical point of the transition at temperatures near 2,000 K and pressures near 120 GPa. Furthermore, we have determined the melting curve of molecular hydrogen up to pressures of 200 GPa, finding a reentrant melting line. The melting line crosses the metalization line at 700 K and 220 GPa using density functional energetics and at 550 K and 290 GPa using quantum Monte Carlo energetics.phase transition | quantum Monte Carlo | density functional theory | plasma phase transition | melting S ince the pioneering work of Wigner and Huntington (1), on the metallization of solid molecular hydrogen by pressure, there has been a great effort to understand the molecular dissociation process in high-pressure hydrogen from both experiment and theory. In the solid, at low temperatures, metallization has been expected to occur in conjunction with a transition to a solid atomic state, although a transition to exotic phases such as quantum fluids (2) or metallic molecular phases may also be possible (3-5).For dense hydrogen in the liquid phase, metallization (probably accompanied by molecular dissociation) can occur either through a continuous process (a crossover) or through a sharp, first-order transition, often called the plasma phase transition. Numerous experiments have been performed in the liquid phase using dynamic compression techniques to measure both the principal Hugoniot in hydrogen and deuterium [using for example: gas guns (6), laser-driven compression (7-9), magnetically driven flyers (10, 11), and converging explosives (12)] and to measure off-Hugoniot properties at lower temperatures [electrical conductivity measurements using shock reverberation (13) and multiple shocks (14), compressibility measurements using explosive-driven generators (15), to mention a few]. The conductivity measurements by Nellis and coworkers (13) produced the first evidence of minimum metallic conductivity in fluid hydrogen at a pressure of 140 GPa and temperatures on the order of 3,000 K. Until recently, there were no experimental indications of a first-order liquid-liquid transition (LLT) in hydrogen. Even though results could not rule out the existence of the transition and most studies had been performed at fairly high temperatures, there was no sign of a sharp, first-order behavior. The only direct experimental evidence of a LLT is from the work of Fortov et al. (15) where reverberating shocks produced with high explosives were used to ramp compress hydrogen, presumably reaching temperatures in the range of 3-8 × 10 3 K. Using highly resolved flash X-ray diagnostics, they were able to measure the compressibility of the liquid and found a 20% increase in density in the regime wher...

It is generally assumed 1,2,3 that solid hydrogen will transform into a metallic alkali-like crystal at sufficiently high pressure. However, some theoretical models 4,5 have also suggested that compressed hydrogen may form an unusual two-component (protons and electrons) metallic fluid at low temperature, or possibly even a zero-temperature liquid ground state. The existence of these new states of matter is conditional on the presence of a maximum in the melting temperature versus pressure curve (the 'melt line'). Previous measurements 6,7,8 of the hydrogen melt line up to pressures of 44 GPa have led to controversial conclusions regarding the existence of this maximum. Here we report ab initio calculations that establish the melt line up to 200 GPa. We predict that subtle changes in the intermolecular interactions lead to a decline of the melt line above 90 GPa. The implication is that as solid molecular hydrogen is compressed, it transforms into a low-temperature quantum fluid before becoming a monatomic crystal. The emerging low-temperature phase diagram of hydrogen and its isotopes bears analogies with the familiar phases of 3 He and 4 He, the only known zero-temperature liquids, but the long-range Coulombic interactions and the large component mass ratio present in hydrogen would ensure dramatically different properties 9,10 .The possible existence of low-temperature liquid phases of compressed hydrogen has been rationalized with arguments based on the nature of effective pair interactions and of the quantum dynamics at high density, resulting in proton-proton correlations insufficient for the stabilization of a crystalline phase 5 . But so far there has been no conclusive evidence establishing whether hydrogen metallizes at low temperature as a solid (the more widely accepted view to date) or as a liquid. Measurements and theoretical predictions of the near-ground state high-pressure phases 3 of hydrogen have proven to be difficult because of the light atomic mass, significant quantum effects and strong electron-ion interactions. In this regard, the finite temperature liquid-solid phase boundary predicted here is especially valuable for understanding the manner in which hydrogen metallizes.The appearance of a maximum melting temperature in hydrogen is in itself a manifestation of an unusual physical phenomenon. The few systems with a negative melt slope involve either open crystalline structures, such as water and graphite, or in the case of closed packed solids, a promotion of valence electrons to higher orbitals upon compression (6s to 5d in cesium 11 , for example). In these cases, the liquid is denser than the solid when they coexist, possibly because of structural or electronic transitions taking place continuously in the liquid, as a function of pressure, but only at discrete pressure intervals in the solid.In contrast, recent experiments 8 have shown that hydrogen phase I -a solid structure with rotationally-free molecules associated with the sites of a hexagonal closed packed (hcp) lattice -persist...

Not all nanopores are created equal. By definition, nanopores have characteristic diameters or conduit widths between ∼1 and 100 nm. However, the narrowest of such pores, perhaps best called Single Digit Nanopores (SDNs) and defined as those with regular diameters less than 10 nm, have only recently been accessible experimentally for precision transport measurements. This Review summarizes recent experiments on pores in this size range that yield surprising results, pointing toward extraordinary transport efficiencies and selectivities for SDN systems. These studies have identified critical gaps in our understanding of nanoscale hydrodynamics, molecular sieving, fluidic structure, and thermodynamics. These knowledge gaps are, in turn, an opportunity to discover and understand fundamentally new mechanisms of molecular and ionic transport at the nanometer scale that may inspire a host of new technologies, from novel membranes for separations and water purification to new gas-permeable materials and energy storage devices. Here we highlight seven critical knowledge gaps in the study of SDNs and identify the need for new approaches to address these topics.

First-principles molecular dynamics simulations have been performed on the solvation of Na+ in water. Consistent with the available experimental data, we find that the first solvation shell of Na+ contains on average 5.2 water molecules. A significant number of water exchanges between the first and second solvation shells are observed. However, the simulations are not long enough to reliably measure the rate of water exchange. Contrary to several previous studies, we do not find any effect of Na+ on the orientation of water molecules outside of the first solvation shell. Furthermore, the complete set of structural properties determined by first-principles molecular dynamics is not predicted by any of the known classical simulations.

A new linear scaling method for computation of the Cartesian Gaussian-based Hartree-Fock exchange matrix is described, which employs a method numerically equivalent to standard direct SCF, and which does not enforce locality of the density matrix. With a previously described method for computing the Coulomb matrix [J. Chem. Phys. 106, 5526 (1997)], linear scaling incremental Fock builds are demonstrated for the first time. Microhartree accuracy and linear scaling are achieved for restricted Hartree-Fock calculations on sequences of water clusters and polyglycine α-helices with the 3-21G and 6-31G basis sets. Eightfold speedups are found relative to our previous method. For systems with a small ionization potential, such as graphitic sheets, the method naturally reverts to the expected quadratic behavior. Also, benchmark 3-21G calculations attaining microhartree accuracy are reported for the P53 tetramerization monomer involving 698 atoms and 3836 basis functions.