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Hydrogen, the simplest and most abundant element in the Universe, develops a remarkably complex behaviour upon compression.sup.1. Since Wigner predicted the dissociation and metallization of solid hydrogen at megabar pressures almost a century ago.sup.2, several efforts have been made to explain the many unusual properties of dense hydrogen, including a rich and poorly understood solid polymorphism.sup.1,3-5, an anomalous melting line.sup.6 and the possible transition to a superconducting state.sup.7. Experiments at such extreme conditions are challenging and often lead to hard-to-interpret and controversial observations, whereas theoretical investigations are constrained by the huge computational cost of sufficiently accurate quantum mechanical calculations. Here we present a theoretical study of the phase diagram of dense hydrogen that uses machine learning to 'learn' potential-energy surfaces and interatomic forces from reference calculations and then predict them at low computational cost, overcoming length- and timescale limitations. We reproduce both the re-entrant melting behaviour and the polymorphism of the solid phase. Simulations using our machine-learning-based potentials provide evidence for a continuous molecular-to-atomic transition in the liquid, with no first-order transition observed above the melting line. This suggests a smooth transition between insulating and metallic layers in giant gas planets, and reconciles existing discrepancies between experiments as a manifestation of supercritical behaviour. Simulations using machine-learning-based interatomic potentials in dense hydrogen overcome system size and timescale limitations, providing evidence of a supercritical behaviour of high-pressure liquid hydrogen and reconciling theoretical and experimental discrepancies., Author(s): Bingqing Cheng [sup.1] [sup.2] [sup.3] , Guglielmo Mazzola [sup.4] , Chris J. Pickard [sup.5] [sup.6] , Michele Ceriotti [sup.7] [sup.8] Author Affiliations: (1) Department of Chemistry, University of Cambridge, [...]

Author(s): Bingqing Cheng [sup.1] , Guglielmo Mazzola [sup.2] , Chris J. Pickard [sup.3] , Michele Ceriotti [sup.4] Author Affiliations: (1) Department of Computer Science and Technology, University of Cambridge, Cambridge, [...]

The quantum nature of the proton can crucially affect the structural and physical properties of hydrogen compounds. For example, in the high-pressure phases (1,2) of [H.sub.2]O, quantum proton fluctuations lead to symmetrization of the hydrogen bond and reduce the boundary between asymmetric and symmetric structures in the phase diagram by 30 gigapascals (ref. 3). Here we show that an analogous quantum symmetrization occurs in the recently discovered (4) sulfur hydride superconductor with a superconducting transition temperature [T.sub.c] of 203 kelvin at 155 gigapascals--the highest [T.sub.c] reported for any superconductor so far. Superconductivity occurs via the formation of a compound with chemical formula [H.sub.3]S (sulfur trihydride) with sulfur atoms arranged on a body-centred cubic lattice (5-9). If the hydrogen atoms are treated as classical particles, then for pressures greater than about 175 gigapascals they are predicted to sit exactly halfway between two sulfur atoms in a structure with Im[bar.3]m symmetry. At lower pressures, the hydrogen atoms move to an off-centre position, forming a short H-S covalent bond and a longer H***S hydrogen bond in a structure with R3m symmetry (5-9). X-ray diffraction experiments confirm the [H.sub.3]S stoichiometry and the sulfur lattice sites, but were unable to discriminate between the two phases (10). Ab initio density-functional-theory calculations show that quantum nuclear motion lowers the symmetrization pressure by 72 gigapascals for [H.sub.3]S and by 60 gigapascals for [D.sub.3]S. Consequently, we predict that the Im[bar.3]m phase dominates the pressure range within which the high [T.sub.c] was measured. The observed pressure dependence of [T.sub.c] is accurately reproduced in our calculations for the Im[bar.3]m phase, but not for the R3m phase. Therefore, the quantum nature of the proton fundamentally changes the superconducting phase diagram of [H.sub.3]S., The recent discovery of high-temperature superconductivity in compressed hydrogen sulfide (4) has led to a number of theoretical studies aimed at understanding the phase diagram of the H-S system and [...]

Little was known about the properties of hydrogen under extreme pressure. Experiments now reveal key details about the arrangement of molecules in several of the element's high-pressure phases. The arrangement of molecules in high-pressure phases of hydrogen., Author(s): Bartomeu Monserrat, Chris J. Pickard Author Affiliations: X-rays glimpse solid hydrogen's structure Hydrogen is the most abundant element in the Universe. Our knowledge of celestial bodies such as the [...]

The gauge-including projector-augmented planewave (GIPAW) density functional theory (DFT) is used to study and describe the NMR chemical shifts in several functionalized single-walled carbon nanotubes (SWNTs). The analysis reveals that a decrease in the degree of functionalization of the system on addition of another group shifts the average chemical shift of the unfunctionalized carbons, but does not affect the shifts of the functionalized carbons.

The calculation of (super 13)C, (super 14)N, (super 15)N, (super 17)O and (super 35)Cl NMR parameters, chemical shift tensors and quadrupolar tensors for (super 14)N, (super 17)O and (super 35)Cl for the crystalline forms of various amino acids is discussed. The isotropic chemical shift value of (super 17)O is markedly affected by small differences in the experimental structure whereas the values of (super 13)C and (super 15)N are relatively insensitive to the differences.

Two-dimensional H-C MAS-J-HMQC solid-state NMR spectra of the two anomeric forms of maltose at natural abundance are presented. The calculated change in the H chemical shift when comparing the full crystal and an isolated molecule is a quantitative measure of intermolecular C--H- - -O weak hydrogen bonding.

The (super 17)O NMR parameters of an amorphous calcium aluminisilicate (CAS) is computed from first-principles. The atomic coordinates of a CAS glass of composition (CaO)0.21(Al2O3)0.12(SiO2)0.67 were obtained by quenching a liquid to room temperature by the means of ab initio molecular-dynamics simulations of the Car-Parrinello type.

The (super 17)O NMR parameters of an amorphous calcium aluminisilicate (CAS) is computed from first-principles. The atomic coordinates of a CAS glass of composition (CaO)0.21(Al2O3)0.12(SiO2)0.67 were obtained by quenching a liquid to room temperature by the means of ab initio molecular-dynamics simulations of the Car-Parrinello type.

The density functional theory and the calculation of the (super 17)O NMR parameters in Ca oxide and Ca aluminosilicates has been presented. We find that the partially covalent nature of the Ca-O bond has a huge impact on the O chemical shifts.

Delta(sub iso), C(sub Q), and n calculations are presented using the density functional theory-based GIPAW method for all silicon, aluminum and oxygen sites and for the 14 O[3] atoms, which are bonded to three Al or Si atoms in the andalusite, sillimanite, and kyanite polymorphs of SiAl2O5. Due to relatively similar electric field gradient and chemical shift values, it is difficult to distinguish the different tricoordinated oxygen atoms present in the structures (OSiAl2 and OAL3) from X-O-X (X=Si, Al) sites by (super 17)) NMR.

We theoretically investigated the local structure of the aperiodic states of zircon (ZrSi[O.sub.4]) using ab initio quantum mechanical calculations. The low and high density liquid and solid glassy phases were obtained by constant volume Car-Parrinello molecular dynamics simulations, using the molar volume of metamict and crystalline zircon, respectively. As in naturally metamict zircons, the polymerization of Si units, the segregation of Zr atoms, and an overall decrease of the Zr coordination were observed. However, the local ordering of our theoretical glasses differs from that of the natural amorphous samples. In the theoretical glasses, the Zr-O distances in the first coordination polyhedra are significantly more distributed and five- and six-coordinated Si species were observed. The relaxation of the glass structure on a time scale exceeding the possibility of molecular modeling is a possible explanation for these discrepancies. The dielectric and (29)Si NMR responses of the glassy phases were computed, providing new constraints for the analysis of experimental data recorded from metamict zircons. The calculated NMR spectra are in good agreement with the experimental NMR spectra in the four-coordinated Si region. Our results show that the usual regular systematic decrease of the (29)Si chemical shift as a function of the polymerization of Si units ([Q.sup.n] species) cannot be used to interpret the (29)Si NMR spectrum of amorphous zircon. In particular, whereas the empirical scale would indicate an average polymerization of [Q.sup.3], the average polymerization of [Q.sup.1.5] observed in the low density zircon glass can account for the experimental data. In a non-uniform model of the structure of metamict zircon, this lower average polymerization is consistent with a clustering of cations limited to few coordination polyhedra.

Periodic density functional theory (DFT) was used to calculate the structure of zircon (ZrSi[O.sub.4]) and reidite (scheelite polymorph of ZrSi[O.sub.4]) at pressures from 0-41 GPa. Subsequently, the [29.sup] Si nuclear magnetic resonance (NMR) chemical shifts at each pressure were determined using the GIPAW method. The results show that in both zircon and reidite the chemical shifts become more negative with increasing pressure. The chemical shift of reidite is predicted by calculation to be -91.8 ppm and measured as -91.1 ppm. The results are used to interpret the previously observed systematic decrease in [sup.29] Si NMR chemical shifts (increasingly negative) in the crystalline fraction of radiation-damaged zircon with increasing [alpha]-dose. The original hypothesis that strain induced by the compression of the crystal by internal regions of damaged, amorphized material at high pressure was the origin of the shifts was disproved. An alternative source of internal pressure in the radiation-damaged crystals linked to '' stuffing' of the lattice with interstitial O or He atoms is proposed.

The high accuracy of the production of the (super 17)O NMR parameters in SiO2 systems is described. The use of this precision to assign the spectrum of the zeolite ferrierite is examined.

The chemical shifts of the N-formyl-L-methionyl-L-leucyl-L-phenylalanine-OMe (MLF) peptide are measured in the solid state and the X-ray structure is used for calculating the [super 13]C chemical shifts by using the gauge including projector augmented wave (GIPAW) method, which has accounted for the periodic nature of the crystal structure. The results have shown that better DFT-based predictions of the chemical shifts of peptides and proteins are achieved by developing improved computational strategies capable of considering the averaging process up to the millisecond time scale on which the chemical shift measurements have reported.

A fast method is described for determining the crystal structure by using crystal structure prediction and solid-state [super 1]H NMR spectroscopy. An ensemble of predicted crystal structures for powdered thymol along with the comparison between experimental and calculated [super 1]H solid-state isotropic NMR chemical shifts is enough to determine which predicted structure has corresponded to the powder under study.

The feasibility of heteronuclear solid-state magic-angle spinning (MAS) NMR experiments for probing [super 15]N-[super 17]O dipolar and J couplings in two isotopically labeled model compounds, glycine*[super 2]HCl and uracil is demonstrated. The results revealed that [super 15]N-[super 17]O solid-state NMR experiments could further be applied to identify specific structure-determining hydrogen-bonding interactions in proteins and nucleic acids.

Solid-state method is applied to calculate J couplings, particularly the [super 15]N-[super 15]N couplings across intra- and intermolecular hydrogen bonds for two molecular crystals. The studies have shown that the differences between the J couplings determined by solution- and solid-state NMR are not due to differences in geometry.

Density functional theory is used for calculating [super 17]O, [super 15]N, [super 13]C, and [super 1]H NMR chemical shieldings in order to differentiate among the three primarily helical forms, [[3.sub.10], [alpha] and [pi] in polyalanine peptides under periodic boundary conditions. The studies have shown [super 17]O as the best observable, as it is sensitive to hydrogen bonding and is affected by small changes in the polypeptide in helix conformations.

A combination of high-resolution solid-state NMR spectroscopy and first-principles calculations is utilized to determine the [super 17]O and [super 29]Si NMR parameters of six different phases of MgSi[O.sub.3].

The temperature dependence of (super 17)O and (super 25)Mg NMR chemicals shifts in solid MgO was calculated by using a first-principles approach. The results of the study showed that the chemical shift behavior with temperature cannot be explained by thermal expansion alone.

An assignment of the experimental (super 17)O NMR shielding parameters for L-glutamic acid-HCI is presented based on first-principles quantum calculations. Some further theoretical calculations on a range of glutamic acid polymorphs are performed and some general conclusions about the influence of hydrogen bonding on (super 17)O NMR shielding parameters are drawn upon.

The results of first-principles calculations of nuclear magnetic resonance (NMR) parameters are presented. The calculations have been performed using the recently introduced gauge including projector augmented wave (GIPAW) method, which was specially devised for periodic systems.

The machine that houses the world's largest laser, and which stands in for the starship Enterprise's warp core in the film Star Trek Into Darkness, has compressed diamond to the [...]

The combination of measured solid-state NMR chemical and shifts and the principles calculations to resolve the crystal structure of (microcrystalline) beta-L-aspartyl-L-alanine is reported. The plane wave DFT based optimization method allows the determination of crystal structures.