1,721,033 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
Liquid–liquid phase transition in hydrogen by coupled electron–ion Monte Carlo simulations
Get PDFThe 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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Path integral Monte Carlo simulations of solid molecular hydrogen surfaces and thin helium-4 films on molecular hydrogen substrates
No full textBased on Richard P. Feynman's formulation of quantum mechanics, Path Integral Monte Carlo is a computational ab-initio method to calculate finite temperature equilibrium properties of quantum many-body systems. As input, only fundamental physical constants and pair-potentials are required. We carry out the first ab-initio particle simulations of three related physical systems. First, the bare H\sb2 substrate is simulated between 0.5 and 1.3K, because a liquid H\sb2 film is a candidate for a new superfluid. We find evidence of quantum exchange in surface terraces for up to 1K. Second, the melting of the H\sb2 surface between 3 and 15K is examined since this is the cleanest example of quantum surface melting. Third, atomically thin superfluid \sp4He films on H\sb2 surfaces are simulated, calculating binding energies per \sp4He atom and third sound, an important experimental probe for superfuid \sp4He films. For all systems we compute density profiles perpendicular and parallel to the surface and compare to experiment. We treat both H\sb2 molecules and \sp4He atoms on the same footing, as spherical particles. For simulations of bulk/vapor interfaces and surface adsorption, a realistic representation of the macroscopic surface is crucial. Therefore, we introduce an external potential to account for arbitrarily layered substrates and long-range corrections. Two algorithms for parallel computers with independent processors are introduced, one to manage concurrent simulations of entire phase-diagrams, and one to improve input/output speed for files shared by all processors.Made available in DSpace on 2011-05-07T12:35:46Z (GMT). No. of bitstreams: 2
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Variational Theory of Hot Dense Matter
No full text112 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2009.We develop a variational theory of hot nuclear matter in neutron stars and supernovae. It can also be used to study charged, hot nuclear matter which may be produced in heavy-ion collisions. This theory is a generalization of the variational theory of cold nuclear and neutron star matter based on realistic models of nuclear forces and pair correlation operators. The present approach uses microcanonical ensembles and the variational principle obeyed by the free energy. We show that the correlated states of the microcanonical ensemble at a given temperature T and density rho can be orthonormalized preserving their diagonal matrix elements of the Hamiltonian. This allows for the minimization of the free energy without corrections from the nonorthogonality of the correlated basis states, similar to that of the ground state energy. Samples of the microcanonical ensemble can be used to study the response, and the neutrino luminosities and opacities of hot matter. We present methods to orthonormalize the correlated states that contribute to the response of hot matter. We apply this variational theory to symmetric nuclear matter and pure neutron matter. This extension generalizes to finite temperatures, the many body technique used in the construction of the zero temperature Akmal-Pandharipande-Ravenhall equation of state. We discuss how the formalism can be used for practical calculations of hot dense matter. Our calculations are a significant improvement over the previous calculation due to Friedman and Pandharipande. The Hamiltonian contains modern realistic two nucleon and three nucleon interactions along with relativistic boost corrections. Expectation values of various operators, including the Hamiltonian, are calculated using cluster expansion and chain summation techniques. The pair correlation operator is now calculated at every density and temperature. Neutral pion condensation along with the associated isovector spin longitudinal sum rule is analyzed. The equation of state is calculated for temperatures less than 30 MeV and densities less than three times the saturation density of nuclear matter. The behavior of the nucleon effective mass in medium is also discussed.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD
Path Integral Monte Carlo Simulations of Helium: From Superfluid Droplets to Quantum Crystals
No full textBelow Tλ = 2.17 K, bulk 4He is a superfluid and has a non-zero Bose-Einstein condensate
fraction. This work will focus primarily on how phenomena such as superfluidity,
Bose condensation and superfluid vortices are manifested in microscopic, inhomogeneous
helium systems. Path Integral Monte Carlo is a powerful method for calculating
the equilibrium properties of quantum systems at finite temperature. We have
achieved linear scaling of computer time with number of particles through the use of
neighbor lists, allowing us to simulate systems of several thousand atoms.
We have derived a local superfluid estimator and used it to examine the microscopic
superfluid response around a molecule rotating in a helium nanodroplet. We
found that the first solvation layer is well-described by a two dimensional superfluid,
with thermal excitations occuring at a lower temperature than in bulk helium. The
effective moment of inertia of a linear impurity in a helium droplet is calculated, and
compared with experimental scattering results. In addition, we calculated the vortex
formation energy for both pure droplets and droplets doped with linear impurities,
and found that the linear impurities had a negligible impact on the formation energy.
A possible spectroscopic signature of vortices in doped helium droplets was suggested.
After deriving a local estimator, we calculated the condensate fraction throughout
the free helium surface of a semi-infinite slab. These results, along with densitydensity
correlation functions, were used to characterize the surface excitations and
calculate the extent to which ripplons are present. In addition, the ripplon dispersion
relation was calculated using imaginary-time correlation functions, and found to be
lll
in good agreement with experimental results.
Finally, we have calculated the Dey be-Waller factor in solid helium for a range of
temperatures and densities, and compared the scaling behavior with the predictions
of harmonic theory. The first non-Gaussian contribution to the density distribution
was calculated.Submitted by William Weathers ([email protected]) on 2012-05-31T19:31:59Z
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Spin Polarization of Ground State Electron Gas at Low Densities
No full textThe electron gas is of great interest in condensed matter physics. It is a simple yet intriguing model that exhibits rich results that help our understanding of the electronic structure of materials. In this work we use Quantum Monte Carlo simulations to study the spontaneous polarization of electron gas at low densities, for both two and three dimensions.
Quantum Monte Carlo is a powerful method to tackle many-body Fermion problems, and its high accuracy has been demonstrated in many previous calculations. The methods we use include variational Monte Carlo and mixed-phase pure diffusion Monte Carlo methods. To construct a high quality wavefunction or density matrix at zero and finite temperature, we apply the Variational
Density Matrix method. We use Random Phase Approximation to derive two-body Jastrow correlation functions. We also include backflow and threebody correlations in the wavefunction, to get a better upper bound for the ground state energies at both variational and fixed-phase level. In
order to reduce the finite size effect at small system sizes, we apply Twist Averaged Boundary Conditions on electron gas. To our knowledge, this is the first application on continuum systems.
With the above methods, we first calculate ground state energies of electron gas
at the variational level. Our study shows that the finite size efect is significantly
reduced with Twist Averaged Boundary Conditions compared to Periodic Boundary
Conditions.
To get a better upper bound in ground state energies, we also perform fixed-phase pure diffusion Monte Carlo calculations on electron gas. With the fixed-phase restriction, an efective potential term comes into the Hamiltonian. We present a cubic polynomial interpolation for an accurate estimation of the path integral of this potential. We perform our calculations with different densities and polarizations, to determine the polarization transition point of ground state electron gas in both two and three dimensions.Submitted by Rachelle Ramer ([email protected]) on 2012-06-05T23:19:16Z
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Quantum Monte Carlo study of zero-point effects in different systems
No full textSubmitted by Meng Tao ([email protected]) on 2012-10-23T18:37:18Z
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