192 research outputs found
Benchmarking vdW‐DF first‐principles predictions against Coupled Electron–Ion Monte Carlo for high‐pressure liquid hydrogen
No full textWe report first-principles results for the nuclear structure and optical responses of high-pressure liquid hydrogen along two isotherms in the region of molecular dissociation. We employ density functional theory with the vdW-DF approximation (vdW) and benchmark the results against existing predictions from Coupled Electron–Ion Monte Carlo (CEIMC). At fixed density and temperature, we find that the pressure obtained from vdW is higher than that from CEIMC by about 10 GPa in the molecular insulating phase and about 20 GPa in the dissociated metallic phase. Molecules are found to be over-stabilized using vdW, with a slightly shorter bond length and with a stronger resistance to compression. As a consequence, pressure dissociation along isotherms using vdW is more progressive than that computed with CEIMC. Below the critical point, the liquid–liquid phase transition is observed with both theories in the same density region, but the one predicted by vdW has a smaller density discontinuity, i.e. a smaller first-order character. The optical conductivity computed using Kubo–Greenwood formulation is rather similar for the two systems and reflects the slightly more pronounced molecular character of vdW
Optical properties of high-pressure fluid hydrogen across molecular dissociation
No full textOptical properties of compressed fluid hydrogen in the region where dissociation and metallization is observed are computed by ab initio methods and compared with recent experimen- tal results. We confirm that at T>3,000 K, both processes are continuous, while at T<1,500 K, the first-order phase transi- tion is accompanied by a discontinuity of the dc conductivity and the thermal conductivity, while both the reflectivity and absorption coefficient vary rapidly but continuously. Our results support the recent analysis of National Ignition Facility (NIF) experiments [Celliers PM, et al. (2018) Science 361:677–682], which assigned the inception of metallization to pressures where the reflectivity is ⇠0.3. Our results also support the conclusion that the temperature plateau seen in laser-heated diamond-anvil cell (DAC) experiments at temperatures higher than 1,500 K corre- sponds to the onset of optical absorption, not to the phase transition. hydroge
Free energy methods in coupled electron ion Monte Carlo
No full textRecent progress in simulation methodologies and in computer power allow first-principles simulations of condensed systems with Born–Oppenheimer electronic energies obtained by quantum Monte Carlo methods. Computing free energies and therefore getting a quantitative determination of phase diagrams is one step more demanding in terms of computer resources. In this paper we derive a general relation to compute the free energy of an ab initio model with Reptation Quantum Monte Carlo (RQMC) energies from the knowledge of the free energy of the same ab initio model in which the electronic energies are computed by the less demanding but less accurate Variational Monte Carlo (VMC) method. Moreover we devise a procedure to correct transition lines based on the use of the new relation. In order to illustrate the procedure, we consider the liquid–liquid phase transition in hydrogen, a first-order transition between a lower pressure, molecular and insulating phase and a higher pressure, partially dissociated and conducting phase. We provide new results along the T = 600 K isotherm across the phase transition and find good agreement between the transition pressure and specific volumes at coexistence for the model with RQMC accuracy between the prediction of our procedure and the values that can be directly inferred from the observed plateau in the pressure–volume curve along the isotherm. This work paves the way for future use of VMC in first-principles simulations of high-pressure hydrogen, an essential simplification when considering larger system sizes or quantum proton effects by Path Integral Monte Carlo methods.ISI
First Principles Methods: A Perspective from Quantum Monte Carlo
No full textQuantum Monte Carlo methods are among the most accurate algorithms for predicting properties of general quantum systems. We briefly introduce ground state, path integral at finite temperature and coupled electron-ion Monte Carlo methods, their merits and limitations. We then discuss recent calculations using these methods for dense liquid hydrogen as it undergoes a molecular/atomic (metal/insulator) transition. We then discuss a procedure that can be used to assess electronic density functionals, which in turn can be used on a larger scale for first principles calculations and apply this technique to dense hydrogen and liquid water
Pairing and entanglement: quantum Monte Carlo studies
No full textDescribed in this dissertation is the use of quantum Monte Carlo methods to study two ideas in quantum many-body problems: superfluidity and entanglement. Density matrices are presented a central tool in the analysis, as are discussed in the review of path integral Monte Carlo (PIMC) and variational Monte Carlo (VMC) methods. PIMC is used to model a one-dimensional system of fermionic lithium atoms according to existing experiments, including a realistic temperature. New estimators of the pair momentum distribution are implemented, yielding in a clear in-situ signature of a pairing mechanism (dubbed FFLO after its first proposers) which implies a microscopic phase fluctuation in space between a normal fluid and a superfluid. VMC is used to model homonuclear diatomic molecules of period-2 elements. The degree of entanglement and the responsible electronic configurations in real space are quantified in terms of the entanglement spectra. Calculating the reduced denstity matrix as an intermediate step reveals a novel way of understanding chemical bonds, as exemplified by Be2 and C2 . Possible implications of these results in integrable many-body models and in quantum chemistry are discussed, as well as direction for future investigation.Submission published under a 24 month embargo labeled 'U of I Access', the embargo will last until 2018-05-01The student, David Yang, accepted the attached license on 2016-04-05 at 03:00.The student, David Yang, submitted this Dissertation for approval on 2016-04-05 at 03:12.This Dissertation was approved for publication on 2016-04-07 at 13:32.DSpace SAF Submission Ingestion Package generated from Vireo submission #9148 on 2016-07-07 at 13:48:51Made available in DSpace on 2016-07-07T20:27:06Z (GMT). No. of bitstreams: 3
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Liquid–liquid phase transition in hydrogen by coupled electron–ion Monte Carlo simulations
No full textThe phase diagram of high-pressure hydrogen is of great interest for fundamental research, planetary physics, and energy applications. A first-order phase transition in the fluid phase between a molecular insulating fluid and a monoatomic metallic fluid has been predicted. The existence and precise location of the transition line is relevant for planetary models. Recent experiments reported contrasting results about the location of the transition. Theoretical results based on density functional theory are also very scattered. We report highly accurate coupled electron–ion Monte Carlo calculations of this transition, finding results that lie between the two experimental predictions, close to that measured in diamond anvil cell experiments but at 25–30 GPa higher pressure. The transition along an isotherm is signaled by a discontinuity in the specific volume, a sudden dissociation of the molecules, a jump in electrical conductivity, and loss of electron localization
A quantum Monte Carlo study of pseudopotentials and metal surfaces
No full textItem marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding ([email protected]) on 2011-05-07T15:01:26Z
Item is restricted indefinitely.Quantum Monte Carlo has been established as a powerful computational tool to study quantum many-body systems. It has been successfully applied to small atoms and molecules, the electron gas, hydrogen at high pressures, silicon and carbon clusters, solid silicon and jellium surfaces. The importance of quantum Monte Carlo for these systems is the very accurate treatment of electronic correlation and in the case of hydrogen the direct treatment of the zero-point motion of protons.In this thesis we propose a method of generating pseudopotentials from correlated wave functions, based on the properties of the one-body density matrix and its natural orbitals. We used quantum Monte Carlo techniques to investigate the influence of electronic correlation in obtaining the one-body density matrix and natural orbitals of lithium, carbon and neon, and their influence in the generation of pseudopotentials.In the second part of this work we applied quantum Monte Carlo methods for the study of highly inhomogeneous systems, namely metal surfaces. We did a study of jellium surfaces at a range of densities representative of metals in Nature. In this work we were concerned to learn more about the nature of the wave function and correlation effects in such systems. Such understanding is very important in the construction of wave functions for real metals and in the development and improvement of approximations used in density functional theory. We present results for electronic densities, pair correlation functions and surfaces energies. The results obtained in such calculations provide important benchmarks for other methods.Made available in DSpace on 2011-05-07T13:58:18Z (GMT). No. of bitstreams: 2
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