Predicted reentrant melting of dense hydrogen at ultra-high pressures

The phase diagram of hydrogen is one of the most important challenges in high-pressure physics and astrophysics. Especially, the melting of dense hydrogen is complicated by dimer dissociation, metallization and nuclear quantum effect of protons, which together lead to a cold melting of dense hydrogen when above 500 GPa. Nonetheless, the variation of the melting curve at higher pressures is virtually uncharted. Here we report that using ab initio molecular dynamics and path integral simulations based on density functional theory, a new atomic phase is discovered, which gives an uplifting melting curve of dense hydrogen when beyond 2 TPa, and results in a reentrant solid-liquid transition before entering the Wigner crystalline phase of protons. The findings greatly extend the phase diagram of dense hydrogen, and put metallic hydrogen into the group of alkali metals, with its melting curve closely resembling those of lithium and sodium.Comment: 27 pages, 10 figure

Vibrational spectroscopy by means of first-principles molecular dynamics simulations

Vibrational spectroscopy is one of the most important experimental techniques for the characterization of molecules and materials. Spectroscopic signatures retrieved in experiments are not always easy to explain in terms of the structure and dynamics of the studied samples. Computational studies are a crucial tool for helping to understand and predict experimental results. Molecular dynamics simulations have emerged as an attractive method for the simulation of vibrational spectra because they explicitly treat the vibrational motion present in the compound under study, in particular in large and condensed systems, subject to complex intramolecular and intermolecular interactions. In this context, first-principles molecular dynamics (FPMD) has been proven to provide an accurate realistic description of many compounds. This review article summarizes the field of vibrational spectroscopy by means of FPDM and highlights recent advances made such as the simulation of Infrared, vibrational circular dichroism, Raman, Raman optical activity, sum frequency generation, and nonlinear spectroscopies

Roadmap on Electronic Structure Codes in the Exascale Era

Electronic structure calculations have been instrumental in providing many important insights into a range of physical and chemical properties of various molecular and solid-state systems. Their importance to various fields, including materials science, chemical sciences, computational chemistry and device physics, is underscored by the large fraction of available public supercomputing resources devoted to these calculations. As we enter the exascale era, exciting new opportunities to increase simulation numbers, sizes, and accuracies present themselves. In order to realize these promises, the community of electronic structure software developers will however first have to tackle a number of challenges pertaining to the efficient use of new architectures that will rely heavily on massive parallelism and hardware accelerators. This roadmap provides a broad overview of the state-of-the-art in electronic structure calculations and of the various new directions being pursued by the community. It covers 14 electronic structure codes, presenting their current status, their development priorities over the next five years, and their plans towards tackling the challenges and leveraging the opportunities presented by the advent of exascale computing.Comment: Submitted as a roadmap article to Modelling and Simulation in Materials Science and Engineering; Address any correspondence to Vikram Gavini ([email protected]) and Danny Perez ([email protected]

Roadmap on Electronic Structure Codes in the Exascale Era

Electronic structure calculations have been instrumental in providing many important insights into a range of physical and chemical properties of various molecular and solid-state systems. Their importance to various fields, including materials science, chemical sciences, computational chemistry and device physics, is underscored by the large fraction of available public supercomputing resources devoted to these calculations. As we enter the exascale era, exciting new opportunities to increase simulation numbers, sizes, and accuracies present themselves. In order to realize these promises, the community of electronic structure software developers will however first have to tackle a number of challenges pertaining to the efficient use of new architectures that will rely heavily on massive parallelism and hardware accelerators. This roadmap provides a broad overview of the state-of-the-art in electronic structure calculations and of the various new directions being pursued by the community. It covers 14 electronic structure codes, presenting their current status, their development priorities over the next five years, and their plans towards tackling the challenges and leveraging the opportunities presented by the advent of exascale computing

Roadmap on Electronic Structure Codes in the Exascale Era

Electronic structure calculations have been instrumental in providing many important insights into a range of physical and chemical properties of various molecular and solid-state systems. Their importance to various fields, including materials science, chemical sciences, computational chemistry and device physics, is underscored by the large fraction of available public supercomputing resources devoted to these calculations. As we enter the exascale era, exciting new opportunities to increase simulation numbers, sizes, and accuracies present themselves. In order to realize these promises, the community of electronic structure software developers will however first have to tackle a number of challenges pertaining to the efficient use of new architectures that will rely heavily on massive parallelism and hardware accelerators. This roadmap provides a broad overview of the state-of-the-art in electronic structure calculations and of the various new directions being pursued by the community. It covers 14 electronic structure codes, presenting their current status, their development priorities over the next five years, and their plans towards tackling the challenges and leveraging the opportunities presented by the advent of exascale computing

Cation-pi Interactions accelerate the living cationic ring-opening polymerization of unsaturated 2-alkyl-2-oxazolines

Cation-dipole interactions were previously shown to have a rate-enhancing effect on the cationic ring-opening polymerization (CROP) of 2-oxazolines bearing a side-chain ester functionality. In line with this, a similar rate enhancement-via intermolecular cation-pi interactions-was anticipated to occur when pi-bonds are introduced into the 2-oxazoline side-chains. Moreover, the incorporation of pi-bonds allows for facile postfunctionalization of the resulting poly(2-oxazoline)s with double and triple bonds in the side-chains via various click reactions. Herein, a combined molecular modeling and experimental approach was used to study the CROP reaction rates of 2-oxazolines with side-chains having varying degrees of unsaturation and side-chain length. The presence of cation-pi interactions and the influence of the degree of unsaturation were initially confirmed by means of regular molecular dynamics simulations on pentameric systems. Furthermore, a combination of enhanced molecular dynamics simulations, static calculations, and a thorough analysis of the noncovalent interactions was performed to unravel to what extent cation-pi interactions alter the reaction kinetics. Additionally, the observed trends were confirmed also in the presence of acetonitrile as solvent, in which experimentally the polymerization is performed. Most intriguingly, we found only a limited effect on the intrinsic reaction kinetics of the CROP and a preorganization effect in the reactive complex region. The latter effect was established by the unsaturated side-chains and the cationic center through a complex interplay between cation-pi, pi-pi, pi-induced dipole, and cation-dipole interactions. These findings led us to propose a two-step mechanism comprised of an equilibration step and a CROP reaction step. The influence of the degree of unsaturation, through a preorganization effect, on the equilibration step was determined with the following trend for the polymerization rates: n-ButylOx = PentynOx. The trend was experimentally confirmed by determining the polymerization rate constants

Mathematical and Numerical Aspects of Quantum Chemistry Problems

This workshop was aimed at strengthtening the interactions between well established experts in quantum chemistry, mathematical analysis, numerical analysis and computational metodology. Most of the mathematicians present in the worskhop have already contributed to the theoretical and numerical study of models in quantum physics and chemistry. Some others, familiar with contiguous fiels, were new to chemistry. Several distinguished researchers in theoretical chemistry participated in the workshop, and presented the mathematical and computational challenges of the field

A quantum Monte Carlo study of high pressure solid and liquid hydrogen

Hydrogen is the first element of the periodic table. As such, it is often regarded as the simplest one: the non-relativistic hydrogen atom is a problem exactly solved in many textbooks; the hydrogen molecular ion H 2 + and the diatomic molecule H 2 are, correspondingly, the first systems to be considered when more than one nucleus is involved. As a thermodynamic system, its phase diagram at low pressures is quite standard: at room temperature and ambient pressure, hydrogen is a molecular fluid; upon cooling, it becomes a molecular solid; its critical point is T=33 K and P=1.3 Pa. Nevertheless, even such a simple system becomes really interesting when pressure is increased by several orders of magnitude. Speculations about the existence of a metallic solid state at 25 GPa and 0 K temperature started with Wigner and Huntington; later calculations suggested that this state could become a high-temperature superconductor . When experiments achieved the predicted transition temperature, they did not find a metallic state; on the other hand, they found a rich phase diagram, where several different solid phases exist. Nowadays, the quest for solid metallic hydrogen at low temperature is still an on-going activity. As temperature is increased above ≈ 1000 K, the system enters the liquid phase: it is important to obtain an accurate equation of state at high temperature and high pressure, in order to model the properties of gas giants, such as Jupiter and Saturn, which are mostly made of hydrogen and helium. Metallic hydrogen, which is yet to be seen in the solid state, was experimentally measured in the liquid phase. Performing experiments at such high pressures is complicated; the information obtained is partial. At low temperatures, the boundaries among the different solid phases can be drawn, but most of their structural properties are still an open problem; at high temperatures, characterizing the insulator-metal transition is hard because of large uncertainties and conflicting results. Ab Initio simulations can be a valuable tool to complement and interpret experimental data; they can also guide experiments with their predictive power. For condensed matter, Density Functional Theory (DFT) is the method of choice to perform Ab Initio simulations at reasonable computational cost. However, their predictive power for high pressure hydrogen is questioned due to several levels of approximation which will be discussed in our work: in particular, the fact that DFT is plagued by an uncontrolled approximation (the exchange-correlation functional approximation) will be elaborated. iiiIn this thesis, we will employ a different method to run Ab Initio simulations of high pressure hydrogen at finite temperature: the Coupled Electron Ion Monte Carlo (CEIMC). We will discuss how CEIMC, combining the Path Integral formalism to treat the nuclear degrees of freedom and the Variational Monte Carlo (VMC) method to accurately compute electronic energies in a Born-Oppenheimer framework, can perform finite temperature simulations without suffering from the same kind of uncontrolled approximation which plagues DFT. We will then apply the method to the low temperature, solid phase and to the high-temperature, liquid one. In the first case, finite temperature simulations of different candidate structures for the various solid phases will be performed, comparing CEIMC results with DFT ones. In the second case, the liquid-liquid phase transition will be investigated, drawing attention to the relationship between molecular dissociation and metallization; to do so, the system will be characterized across the transition with the computation of relevant optical properties