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We present an overview of the variational and diffusion quantum Monte Carlo methods as implemented in the casino program. We particularly focus on developments made in the last decade, describing state-of-the-art quantum Monte Carlo algorithms and software and discussing their strengths and weaknesses. We review a range of recent applications of casino. [ABSTRACT FROM AUTHOR]

We report all-electron and pseudopotential calculations of the ground-state energies of the neutral Ne atom and the Ne+ ion using the variational and diffusion quantum Monte Carlo (DMC) methods. We investigate different levels of Slater-Jastrow trial wave function: (i) using Hartree-Fock orbitals, (ii) using orbitals optimized within a Monte Carlo procedure in the presence of a Jastrow factor, and (iii) including backflow correlations in the wave function. Small reductions in the total energy are obtained by optimizing the orbitals, while more significant reductions are obtained by incorporating backflow correlations. We study the finite-time-step and fixed-node biases in the DMC energy and show that there is a strong tendency for these errors to cancel when the first ionization potential (IP) is calculated. DMC gives highly accurate values for the IP of Ne at all the levels of trial wave function that we have considered. [ABSTRACT FROM AUTHOR]

We report variational and diffusion quantum Monte Carlo (VMC and DMC) calculations of the dissociation energies of the three-electron hemibonded radical cationic dimers of He, NH3, H2O, HF, and Ne. These systems are particularly difficult for standard density-functional methods such as the local-density approximation and the generalized gradient approximation. We have performed both all-electron (AE) and pseudopotential (PP) calculations using Slater-Jastrow wave functions with Hartree-Fock single-particle orbitals. Our results are in good agreement with coupled-cluster CCSD(T) calculations. We have also studied the relative stability of the hemibonded and hydrogen-bonded water radical dimer isomers. Our calculations indicate that the latter isomer is more stable, in agreement with post-Hartree-Fock methods. The excellent agreement between our AE and PP results demonstrates the high quality of the PPs used within our VMC and DMC calculations. [ABSTRACT FROM AUTHOR]

A simple scheme is described for introducing the correct cusps at nuclei into orbitals obtained from Gaussian basis set electronic structure calculations. The scheme is tested with all-electron variational quantum Monte Carlo (VMC) and diffusion quantum Monte Carlo (DMC) methods for the Ne atom, the H2 molecule, and 55 molecules from a standard benchmark set. It greatly reduces the variance of the local energy in all cases and slightly improves the variational energy. This scheme yields a general improvement in the efficiency of all-electron VMC and DMC calculations using Gaussian basis sets. [ABSTRACT FROM AUTHOR]

We report diffusion quantum Monte Carlo (DMC) calculations of the quasiparticle and excitonic gaps of hydrogen-terminated oligoynes and extended polyyne. The electronic gaps are found to be very sensitive to the atomic structure in these systems. We have therefore optimised the geometry of polyyne by directly minimising the DMC energy with respect to the lattice constant and the Peierls-induced carbon–carbon bond-length alternation. We find the bond-length alternation of polyyne to be 0.136(2) Å and the excitonic and quasiparticle gaps to be 3.30(7) and 3.4(1) eV, respectively. The DMC zone-centre longitudinal optical phonon frequency of polyyne is 2084(5) cm−1, which is consistent with Raman spectroscopic measurements for large oligoynes. [ABSTRACT FROM AUTHOR]

We report diffusion quantum Monte Carlo calculations of the interlayer binding energy of bilayer graphene. We find the binding energies of the AA-and AB-stacked structures at the equilibrium separation to be 11.5(9) and 17.7(9) meV/atom, respectively. The out-of-plane zone-center optical phonon frequency predicted by our binding-energy curve is consistent with available experimental results. As well as assisting the modeling of interactions between graphene layers, our results will facilitate the development of van der Waals exchange-correlation functionals for density functional theory calculations. [ABSTRACT FROM AUTHOR]

We use density functional theory to calculate the electronic band structures, cohesive energies, phonon dispersions, and optical absorption spectra of two-dimensional In2X2 crystals, where X is S, Se, orTe. We identify two crystalline phases (α and β) of monolayers of hexagonal In2X2, and show that they are characterized by different sets of Raman-active phonon modes. We find that these materials are indirect-band-gap semiconductors with a sombrero-shaped dispersion of holes near the valence-band edge. The latter feature results in a Lifshitz transition (a change in the Fermi-surface topology of hole-doped In2X2) at hole concentrations ns = 6.86 × 1013 cm-2, nSe = 6.20 × 1013 cm-2, and nTe = 2.86 × 1013 cm-2 for X=S, Se, and Te, respectively, for α-In2X2 and ns = 8.32 × 1013 cm-2, nSe = 6.00 × 1013 cm-2, and nTe = 8.14 × 1013 cm-2 for β-In2X2. [ABSTRACT FROM AUTHOR]

Solid He is studied in the pressure and temperature ranges 1-40 TPa and 0-10 000 K using first-principles methods. Anharmonic vibrational properties are calculated within a self-consistent field framework, including the internal and free energies, density-pressure relation, stress tensor, thermal expansion, and the electron-phonon coupling renormalization of the electronic band gap. We find that an accurate description of electron-phonon coupling requires us to use a nonperturbative approach. The metallization pressure of 32.9 TPa at 0 K is larger than found previously. The vibrational effects are large; for example, at P=30 TPa the band gap is increased by 2.8 eV by electron-phonon coupling and a further 0.1 eV by thermal expansion compared to the static value. The implications of the calculated metallization pressure for the cooling of white dwarfs are discussed. [ABSTRACT FROM AUTHOR]

Fermi liquid theory is the basic paradigm within which we understand the normal behavior of interacting electron systems, but quantitative values for the parameters that occur in this theory are currently unknown in many important cases. One such case is the two-dimensional homogeneous electron gas (2D HEG), which is realized in a wide variety of semiconductor devices. We have used quantum Monte Carlo (QMC) methods to calculate the Landau interaction functions between pairs of quasiparticles. We use these to study the Fermi liquid parameters, finding that finite-size effects represent a serious obstacle to the direct determination of Fermi liquid parameters in QMC calculations. We have used QMC data in the literature for other properties of the 2D HEG to assemble a set of "best available" values for the Fermi liquid parameters. [ABSTRACT FROM AUTHOR]

We report density-functional-theory calculations of the electronic band structures and optical absorption spectra of two-dimensional crystals of Ga2X2 (X = S. Se, and Te). Our calculations show that all three two-dimensional materials are dynamically stable indirect-band-gap semiconductors with a sombrero dispersion of holes near the top of the valence band. We predict the existence of Lifshitz transitions—changes in the Fermi-surface topology of hole-doped Ga2X2—at hole concentrations nS = 7.96 × 1013 cm-2, nSe = 6.13 × 1013 cm-2, and nTe = 3.54 × 1013 cm-2. [ABSTRACT FROM AUTHOR]

A unified approach is used to study vibrational properties of periodic systems with first-principles methods and including anharmonic effects. Our approach provides a theoretical basis for the determination of phonon-dependent quantities at finite temperatures. The low-energy portion of the Born-Oppenheimer energy surface is mapped and used to calculate the total vibrational energy including anharmonic effects, electron-phonon coupling, and the vibrational contribution to the stress tensor. We report results for the temperature dependence of the electronic band gap and the linear coefficient of thermal expansion of diamond, lithium hydride, and lithium deuteride. [ABSTRACT FROM AUTHOR]

The quasiparticle effective mass is a key quantity in the physics of electron gases, describing the renormalization of the electron mass due to electron-electron interactions. Two-dimensional electron gases are of fundamental importance in semiconductor physica, and there have been numerous experimental and theoretical attempts to determine the quasiparticle effective mass in these systems. In this work we report quantum Monte Carlo results for the quasiparticle effective mass of a two-dimensional homogeneous electron gas. Our calculations differ from previous quantum Monte Carlo work in that much smaller statistical error bars have been achieved, allowing for an improved treatment of finite-size effects. In some cases we have also been able to use larger system sizes than previous calculations. [ABSTRACT FROM AUTHOR]

We report calculations of the electronic structure of silicene and the stability of its weakly buckled honeycomb lattice in an external electric field oriented perpendicular to the monolayer of Si atoms. The electric field produces a tunable band gap in the Dirac-type electronic spectrum, the gap being suppressed by a factor of about eight by the high polarizability of the system. At low electric fields, the interplay between this tunable band gap, which is specific to electrons on a honeycomb lattice, and the Kane-Mele spin-orbit coupling induces a transition from a topological to a band insulator, whereas at much higher electric fields silicene becomes a semimetal [ABSTRACT FROM AUTHOR]

Quantum Monte Carlo calculations of the relaxation energy, pair-correlation function, and annihilating-pair momentum density are presented for a positron immersed in a homogeneous electron gas. We find smaller relaxation energies and contact pair-correlation functions in the important low-density regime than predicted by earlier studies. Our annihilating-pair momentum densities have almost zero weight above the Fermi momentum due to the cancellation of electron-electron and electron-positron correlation effects. [ABSTRACT FROM AUTHOR]

Motivated by potential realizations in cold-atom or cold-molecule systems, we have performed quantum Monte Carlo simulations of two-component gases of fermions in two dimensions with hard-core interactions. We have determined the gross features of the zero-temperature phase diagram by investigating the relative stabilities of paramagnetic and ferromagnetic fluids and crystals. We have also examined the effect of including a pairwise, long-range r-3 potential between the particles. Our most important conclusion is that there is no region of stability for a ferromagnetic fluid phase, even if the long-range interaction is present. We also present results for the pair-correlation function, static structure factor, and momentum density of two-dimensional hard-core fluids. [ABSTRACT FROM AUTHOR]

The objectives of the study were to identify, compare and contrast the expectations of key stakeholders regarding cholinesterase inhibitor treatments prescribed to people with Alzheimer's Disease (AD), and to examine those expectations in the light of recent findings regarding the efficacy and effectiveness of this class of drug. Participants were four older women diagnosed with early stage AD, their lay caregivers, their professional healthcare providers, and pharmacists and policy makers, making a total of 23 participants. Semi-structured interviews were performed in a conversational style designed to elicit accounts of expectations of cholinesterase inhibitors in relation to diagnosis and treatment for AD. The interviews were carried out in the participants' own homes, their places of work or by telephone. A five-member, multidisciplinary team taped, transcribed and coded the interviews using qualitative software in order to identify overarching themes.The study found that although the policy makers and physicians harbored more skeptical or negative expectations, the majority of stakeholders expected that people diagnosed with AD would reap beneficial effects through the use of cholinesterase inhibitors. The study concludes that there is still controversy about the use of cholinesterase inhibitors, particularly in terms of their cost-effectiveness. A clear and ethical argument exists in support of attempts to both modify unrealistic patient expectations where they exist, and to enhance the knowledge base of prescribers. Greater concordance between physicians', patients' and caregivers' treatment goals, and appropriate prescription of these agents in line with available research will require more complete patient and caregiver information on the one hand, and potentially a shift in emphasis to more individualized, clinically focused outcomes, on the other. [ABSTRACT FROM AUTHOR]

OBJECTIVE: To develop a typology of after-hours care (AHC) instructions and to examine physician and practice characteristics associated with each type of instruction.DESIGN: Cross-sectional telephone survey. Physicians' offices were called during evenings and weekends to listen to their messages regarding AHC. All messages were categorized. Thematic analysis of a subset of messages was conducted to develop a typology of AHC instructions. Logistic regression analysis was used to identify associations between physician and practice characteristics and the instructions left for patients.SETTING: Family practices in the greater Toronto area.PARTICIPANTS: Stratified random sample of family physicians providing office-based primary care.MAIN OUTCOME MEASURES: Form of response (eg, answering machine), content of message, and physician and practice characteristics.RESULTS: Of 514 after-hours messages from family physicians' offices, 421 were obtained from answering machines, 58 were obtained from answering services, 23 had no answer, 2 gave pager numbers, and 10 had other responses. Message content ranged from no AHC instructions to detailed advice; 54% of messages provided a single instruction, and the rest provided a combination of instructions. Content analysis identified 815 discrete instructions or types of response that were classified into 7 categories: 302 instructed patients to go to an emergency department; 122 provided direct contact with a physician; 115 told patients to go to a clinic; 94 left no directions; 76 suggested calling a housecall service; 45 suggested calling Telehealth; and 61 suggested other things. About 22% of messages only advised attending an emergency department, and 18% gave no advice at all. Physicians who were female, had Canadian certification in family medicine, held hospital privileges, or had attended a Canadian medical school were more likely to be directly available to their patients.CONCLUSION: Important issues identified included the recommendation to use an emergency department as the sole source of AHC, practices providing no specific AHC instructions to their patients, and physicians' lack of acceptance of Telehealth. To improve AHC, new initiatives should build upon the existing system, changes should be integrated, and there should be a range of AHC options for patients and physicians. [ABSTRACT FROM AUTHOR]

OBJECTIVE: To develop a taxonomy of errors derived solely from the content of error reports using Canadian data from the Primary Care International Study of Medical Errors.DESIGNSecondary analysis of data from a descriptive, cross-sectional, self-report survey.SETTING: Community-based family medicine clinics.PARTICIPANTS: Family physicians.INTERVENTION: Implementation of an error-reporting system for family medicine.MAIN OUTCOME MEASURES: Type of error, type of causal factor.RESULTS: Six types of errors or adverse events (administrative, communication, diagnostic, documentation, medication, and surgical or procedural) and 10 causal factors (case complexity, discontinuity of care, failure to follow protocol or accepted practice, fatigue, gap in knowledge, high workload, insufficient information on pharmacologic properties of medication, medication side effects, relationship dynamics, and structural problems) were identified.CONCLUSION: Our taxonomy differs from that adopted by the Primary Care International Study of Medical Errors. We propose that our taxonomy is better suited for the purposes of family physicians reporting errors in Canada. [ABSTRACT FROM AUTHOR]

Assessment tools are a vital component of chronic disease management. The Royal College of Physicians has developed a patient-focused outcome measure for the treatment of asthma, the 'Three Key Questions'. However, in a study investigating the goals of people with asthma, several issues related to the tool emerged. Forty-seven adults of a range of ages and asthma severity but with no significant co-morbidity were interviewed. It emerged that the outcome measuer may be subject to recall bias. Also, symptom reports may be conflated if daytime symptoms also occur with activity. 'Interference with activity' is a subjective term the interpretation of which varies considerably. Changes in the level of activity undertaken may be reported rather than changes in symptom severity. The 'Three Key Questions' are not fully patient-centred because they assess the presence of symptoms rather than their importance to the individual. The use of the 'Three Key Questions' as an outcome measure may not allow valid comparisons to be made between settings. [ABSTRACT FROM AUTHOR]

According to Landau's Fermi liquid theory, the main properties of the quasiparticle excitations of an electron gas are embodied in the effective mass m*, which determines the energy of a single quasiparticle, and the Landau interaction function, which indicates how the energy of a quasiparticle is modified by the presence of other quasiparticles. This simple paradigm underlies most of our current understanding of the physical and chemical behavior of metallic systems. The quasiparticle effective mass of the three-dimensional homogeneous electron gas has been the subject of theoretical controversy, and there is a lack of experimental data. In this Letter, we deploy diffusion Monte Carlo (DMC) methods to calculate m* as a function of density for paramagnetic and ferromagnetic three-dimensional homogeneous electron gases. The DMC results indicate that m* decreases when the density is reduced, especially in the ferromagnetic case. The DMC quasiparticle energy bands exclude the possibility of a reduction in the occupied bandwidth relative to that of the free-electron model at density parameter rs=4, which corresponds to Na metal. [ABSTRACT FROM AUTHOR]

A correction to the article "Quantum Monte Carlo study of the Ne atom and the Ne+ Ion" that was published in the 2006 issue is presented.

We present an accurate study of the static-nucleus electronic energy band gap of solid molecular hydrogen at high pressure. The excitonic and quasiparticle gaps of the C2/c, Pc, Pbcn, and P63/m structures at pressures of 250, 300, and 350 GPa are calculated using the fixed-node diffusion quantum Monte Carlo (DMC) method. The difference between the mean-field and many-body band gaps at the same density is found to be almost independent of system size and can therefore be applied as a scissor correction to the mean-field gap of an infinite system to obtain an estimate of the many-body gap in the thermodynamic limit. By comparing our static-nucleus DMC energy gaps with available experimental results, we demonstrate the important role played by nuclear quantum effects in the electronic structure of solid hydrogen. [ABSTRACT FROM AUTHOR]

We report a systematic analysis of the performance of a widely used set of Dirac-Fock pseudopotentials for quantum Monte Carlo (QMC) calculations. We study each atom in the periodic table from hydrogen (Z=1) to mercury (Z=80), with the exception of the 4f elements (57≤Z≤70). We demonstrate that ghost states are a potentially serious problem when plane-wave basis sets are used in density functional theory (DFT) orbital-generation calculations, but that this problem can be almost entirely eliminated by choosing the s channel to be local in the DFT calculation; the d channel can then be chosen to be local in subsequent QMC calculations, which generally leads to more accurate results. We investigate the achievable energy variance per electron with different levels of trial wave function and we determine appropriate plane-wave cutoff energies for DFT calculations for each pseudopotential. We demonstrate that the so-called "T-move" scheme in diffusion Monte Carlo is essential for many elements. We investigate the optimal choice of spherical integration rule for pseudopotential projectors in QMC calculations. The information reported here will prove crucial in the planning and execution of QMC projects involving beyond-first-row elements. [ABSTRACT FROM AUTHOR]

Abstract During routine sampling of bulk raw milk on a dairy farm, the pathogenic bacteria Listeria monocytogenes was found to be a contaminant, at numbers L. monocytogenes, indicating a possible case of excretion of the L. monocytogenes directly into the milk. Milk samples were collected from the individual cows and analysed, resulting in the identification of L. monocytogenes excretion (at 280 cfu/ml) from one of the 4 mammary quarters of one dairy cow out of 180. When the infected cow was isolated from the herd, no L. monocytogenes was detected from the remaining herd. The pulsed-field gel electrophoresis pattern of the strain from the individual cow was indistinguishable from that originally isolated from the bulk milk. The infected cow did not show any clinical signs of disease, nor did the appearance of the milk have any physical abnormalities. Antibiotic treatment of the infected mammary quarter was found to be ineffective. This study shows that there can be risks associated with direct contamination of raw milk with L. monocytogenes.

We have studied the spin-polarized three-dimensional homogeneous electron gas using the diffusion quantum Monte Carlo method, with trial wave functions including backflow and three-body correlations in the Jastrow factor, and we have used twist averaging to reduce finite-size effects. Calculations of the pair-correlation function, including the on-top pair density, as well as the structure factor and the total energy, are reported for systems of 118 electrons in the density range rs=0.5-20 a.u., and for spin polarizations of 0, 0.34, 0.66, and 1. We consider the spin resolution of the pair-correlation function and structure factor, and the energy of spin polarization. We show that a control variate method can reduce the variance when twist averaging, and we have achieved higher accuracy and lower noise than earlier quantum Monte Carlo studies. [ABSTRACT FROM AUTHOR]

We have developed a flexible framework for constructing Jastrow factors which allows for the introduction of terms involving arbitrary numbers of particles. The use of various three- and four-body Jastrow terms in quantum Monte Carlo calculations is investigated, including a four-body van der Waals-like term, and anisotropic terms. We have tested these Jastrow factors on one- and two-dimensional homogeneous electron gases, the Be, B, and Ο atoms, and the BeH, H20, N2, and H2 molecules. Our optimized Jastrow factors retrieve more than 90% of the fixed-node diffusion Monte Carlo correlation energy in variational Monte Carlo for each system studied [ABSTRACT FROM AUTHOR]

We compute the absorption and emission energies and hence Stokes shifts of small diamondoids as a function of size using different theoretical approaches, including density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations. The absorption spectra of these molecules are also investigated by time-dependent DFT and compared with experiment. We analyze the structural distortion and formation of a self-trapped exciton in the excited state, and we study the effects of these on the Stokes shift as a function of size. Compared to recent experiments, QMC overestimates the excitation energies by about 0.8(1) eV on average. Benefiting from a cancellation of errors, the optical gaps obtained in DFT calculations with the B3LYP functional are in better agreement with experiment. It is also shown that time-dependent B3LYP calculations can reproduce most of the features found in the experimental spectra. According to our calculations, the structures of diamondoids in the excited state show a distortion which is hardly noticeable compared to that found for methane. As the number of diamond cages is increased, the distortion mechanism abruptly changes character. We have shown that the Stokes shift is size dependent and decreases with the number of diamond cages. If we neglect orbital symmetry effects on the optical excitations, the rate of decrease in the Stokes shift is, on average, 0.1 eV per cage for small diamondoids. [ABSTRACT FROM AUTHOR]

Excitonic effects play a particularly important role in the optoelectronic behavior of two-dimensional (2D) semiconductors. To facilitate the interpretation of experimental photoabsorption and photoluminescence spectra we provide statistically exact diffusion quantum Monte Carlo binding-energy data for Mott-Wannier models of excitons, trions, and biexcitons in 2D semiconductors. We also provide contact pair densities to allow a description of contact (exchange) interactions between charge carriers using first-order perturbation theory. Our data indicate that the binding energy of a trion is generally larger than that of a biexciton in 2D semiconductors. We provide interpolation formulas giving the binding energy and contact density of 2D semiconductors as functions of the electron and hole effective masses and the in-plane polarizability. [ABSTRACT FROM AUTHOR]

The interaction between a single hole and a two-dimensional, paramagnetic, homogeneous electron gas is studied using diffusion quantum Monte Carlo simulations. Electron-hole relaxation energies, pair-correlation functions, and electron-hole center-of-mass momentum densities are reported for a range of electron-hole mass ratios and electron densities. We find numerical evidence of a crossover from a collective excitonic state to a trion-dominated state in a density range in agreement with that found in recent experiments on quantum-well heterostructures. [ABSTRACT FROM AUTHOR]

We describe a number of strategies for minimizing and calculating accurately the statistical uncertainty in quantum Monte Carlo calculations. We investigate the impact of the sampling algorithm on the efficiency of the variational Monte Carlo method. We then propose a technique to maximize the efficiency of the linear extrapolation of diffusion Monte Carlo results to zero time step, finding that a relative time-step ratio of 1:4 is optimal. Finally, we discuss the removal of serial correlation from data sets by reblocking, setting out criteria for the choice of block length and quantifying the effects of the uncertainty in the estimated correlation length. [ABSTRACT FROM AUTHOR]

Excitonic effects play a particularly important role in the optoelectronic behavior of two-dimensional semiconductors. To facilitate the interpretation of experimental photoabsorption and photoluminescence spectra we provide (i) statistically exact diffusion quantum Monte Carlo binding-energy data for a Mott-Wannier model of (donor/acceptor-bound) excitons, trions, and biexcitons in two-dimensional semiconductors in which charges interact via the Keldysh potential, (ii) contact pair-distribution functions to allow a perturbative description of contact interactions between charge carriers, and (iii) an analysis and classification of the different types of bright trions and biexcitons that can be seen in single-layer molybdenum and tungsten dichalcogenides. We investigate the stability of biexcitons in which two charge carriers are indistinguishable, finding that they are only bound when the indistinguishable particles are several times heavier than the distinguishable ones. Donor/acceptor-bound biexcitons have similar binding energies to the experimentally measured biexciton binding energies. We predict the relative positions of all stable free and bound excitonic complexes of distinguishable charge carriers in the photoluminescence spectra of WSe2 and MoSe2. [ABSTRACT FROM AUTHOR]