41 research outputs found
Local Variational Principle
A generalization of the Gibbs-Bogoliubov-Feynman inequality for spinless
particles is proven and then illustrated for the simple model of a symmetric
double-well quartic potential. The method gives a pointwise lower bound for the
finite-temperature density matrix and it can be systematically improved by the
Trotter composition rule. It is also shown to produce groundstate energies
better than the ones given by the Rayleigh-Ritz principle as applied to the
groundstate eigenfunctions of the reference potentials. Based on this
observation, it is argued that the Local Variational Principle performs better
than the equivalent methods based on the centroid path idea and on the
Gibbs-Bogoliubov-Feynman variational principle, especially in the range of low
temperatures.Comment: 15 pages, 5 figures, one more section adde
Transfer-Matrix Monte Carlo Estimates of Critical Points in the Simple Cubic Ising, Planar and Heisenberg Models
The principle and the efficiency of the Monte Carlo transfer-matrix algorithm
are discussed. Enhancements of this algorithm are illustrated by applications
to several phase transitions in lattice spin models. We demonstrate how the
statistical noise can be reduced considerably by a similarity transformation of
the transfer matrix using a variational estimate of its leading eigenvector, in
analogy with a common practice in various quantum Monte Carlo techniques. Here
we take the two-dimensional coupled -Ising model as an example.
Furthermore, we calculate interface free energies of finite three-dimensional
O() models, for the three cases , 2 and 3. Application of finite-size
scaling to the numerical results yields estimates of the critical points of
these three models. The statistical precision of the estimates is satisfactory
for the modest amount of computer time spent
Introduction to Configuration Path Integral Monte Carlo
In low-temperature high-density plasmas quantum effects of the electrons are
becoming increasingly important. This requires the development of new
theoretical and computational tools. Quantum Monte Carlo methods are among the
most successful approaches to first-principle simulations of many-body quantum
systems. In this chapter we present a recently developed method---the
configuration path integral Monte Carlo (CPIMC) method for moderately coupled,
highly degenerate fermions at finite temperatures. It is based on the second
quantization representation of the -particle density operator in a basis of
(anti-)symmetrized -particle states (configurations of occupation numbers)
and allows to tread arbitrary pair interactions in a continuous space.
We give a detailed description of the method and discuss the application to
electrons or, more generally, Coulomb-interacting fermions. As a test case we
consider a few quantum particles in a one-dimensional harmonic trap. Depending
on the coupling parameter (ratio of the interaction energy to kinetic energy),
the method strongly reduces the sign problem as compared to direct path
integral Monte Carlo (DPIMC) simulations in the regime of strong degeneracy
which is of particular importance for dense matter in laser plasmas or compact
stars. In order to provide a self-contained introduction, the chapter includes
a short introduction to Metropolis Monte Carlo methods and the second
quantization of quantum mechanics.Comment: chapter in book "Introduction to Complex Plasmas: Scientific
Challenges and Technological Opportunities", Michael Bonitz, K. Becker, J.
Lopez and H. Thomsen (Eds.) Springer Series "Atomic, Optical and Plasma
Physics", vol. 82, Springer 2014, pp. 153-194 ISBN: 978-3-319-05436-0 (Print)
978-3-319-05437-7 (Online
The Cholecystectomy As A Day Case (CAAD) Score: A Validated Score of Preoperative Predictors of Successful Day-Case Cholecystectomy Using the CholeS Data Set
Background
Day-case surgery is associated with significant patient and cost benefits. However, only 43% of cholecystectomy patients are discharged home the same day. One hypothesis is day-case cholecystectomy rates, defined as patients discharged the same day as their operation, may be improved by better assessment of patients using standard preoperative variables.
Methods
Data were extracted from a prospectively collected data set of cholecystectomy patients from 166 UK and Irish hospitals (CholeS). Cholecystectomies performed as elective procedures were divided into main (75%) and validation (25%) data sets. Preoperative predictors were identified, and a risk score of failed day case was devised using multivariate logistic regression. Receiver operating curve analysis was used to validate the score in the validation data set.
Results
Of the 7426 elective cholecystectomies performed, 49% of these were discharged home the same day. Same-day discharge following cholecystectomy was less likely with older patients (OR 0.18, 95% CI 0.15–0.23), higher ASA scores (OR 0.19, 95% CI 0.15–0.23), complicated cholelithiasis (OR 0.38, 95% CI 0.31 to 0.48), male gender (OR 0.66, 95% CI 0.58–0.74), previous acute gallstone-related admissions (OR 0.54, 95% CI 0.48–0.60) and preoperative endoscopic intervention (OR 0.40, 95% CI 0.34–0.47). The CAAD score was developed using these variables. When applied to the validation subgroup, a CAAD score of ≤5 was associated with 80.8% successful day-case cholecystectomy compared with 19.2% associated with a CAAD score >5 (p < 0.001).
Conclusions
The CAAD score which utilises data readily available from clinic letters and electronic sources can predict same-day discharges following cholecystectomy