Biomedical subjects

Henry Krakauer

Publications and source records attributed to Henry Krakauer.

7 recordsLinked to original sources

PubMed2006

Auxiliary-field quantum Monte Carlo study of first- and second-row post-d elements.

A series of calculations for the first- and second-row post-d elements (Ga-Br and In-I) are presented using the phaseless auxiliary-field quantum Monte Carlo (AF QMC) method. This method is formulated in a Hilbert space defined by any chosen one-particle basis and maps the many-body problem into a linear combination of independent-particle solutions with external auxiliary fields. The phase/sign problem is handled approximately by the phaseless formalism using a trial wave function, which in our calculations was chosen to be the Hartree-Fock solution. We used the consistent correlated basis sets of Peterson et al. [J. Chem. Phys. 119, 11099 (2003); 119, 11113 (2003)], which employ a small-core relativistic pseudopotential. The AF QMC results are compared with experiment and with those from density functional (generalized gradient approximation and B3LYP) and CCSD(T) calculations. The AF QMC total energies agree with CCSD(T) to within a few millihartrees across the systems and over several basis sets. The calculated atomic electron affinities, ionization energies, and spectroscopic properties of dimers are, at large basis sets, in excellent agreement with experiment.

Journal Article↗

PubMed2006

Auxiliary-field quantum Monte Carlo calculations of molecular systems with a Gaussian basis.

We extend the recently introduced phaseless auxiliary-field quantum Monte Carlo (QMC) approach to any single-particle basis and apply it to molecular systems with Gaussian basis sets. QMC methods in general scale favorably with the system size as a low power. A QMC approach with auxiliary fields, in principle, allows an exact solution of the Schrodinger equation in the chosen basis. However, the well-known sign/phase problem causes the statistical noise to increase exponentially. The phaseless method controls this problem by constraining the paths in the auxiliary-field path integrals with an approximate phase condition that depends on a trial wave function. In the present calculations, the trial wave function is a single Slater determinant from a Hartree-Fock calculation. The calculated all-electron total energies show typical systematic errors of no more than a few millihartrees compared to exact results. At equilibrium geometries in the molecules we studied, this accuracy is roughly comparable to that of coupled cluster with single and double excitations and with noniterative triples [CCSD(T)]. For stretched bonds in H(2)O, our method exhibits a better overall accuracy and a more uniform behavior than CCSD(T).

Journal Article↗

PubMed2005

Assessing and using the multiple correlated components of the burden of disease in decision-making in health care.

In medical decision-making, we must (a) establish the patient's prognosis, and (b) identify the therapeutic strategy that will best improve that prognosis. The prognosis is the projected progression over time of the cumulative probability of (a) mortality, (b) morbidity, (c) disability, (d) psychological distress, and (e) resource use. Our modeling employs parametric representations and multivariate analysis to produce the predicted individual and joint probabilities of any range of each measure and of combinations of the measures. The predicted probabilities are consistent with their observed values. We have demonstrated, we believe, the feasibility of analyzing and predicting the burden of disease as a means to support decision-making at the bedside and the policy level.

Cost of Illness↗

PubMed2005

Projected survival benefit as criterion for listing and organ allocation in heart transplantation.

BACKGROUND: Current policies for the selection of candidates and the allocation of hearts for transplantation give priority to patients at greatest risk if not transplanted. However, to achieve best use of the donated organs, it is necessary to estimate the net benefit associated with transplantation. METHODS: The survival benefit associated with being listed or not, with being transplanted or left on the waiting list, or with being transplanted or being denied the opportunity for a transplant can be estimated by means of time-to-event modeling of competing risks with intervening states. The data were obtained from the Organ Procurement and Transplantation Network and describe the outcomes of listings from 1997 to June 1999. Our analyses assessed 9,059 heart transplantation candidates, who were followed for at least 1 and up to 2 years after listing. RESULTS: The probability of receiving a heart transplant does not increase with the probability of death while awaiting the transplant. It is comparable in the second and tenth deciles of risk (measured by the probability of death while awaiting a transplant) at 1 month after listing (15% vs 18%), but is considerably higher in the second decile at 6 months (53% vs 38%) and increasingly more so thereafter. The estimate of survival benefit stabilizes within 1 year of follow-up. Through the fourth decile of risk, the benefit of being placed on the waiting list is negligible at best, but becomes substantial (10%) for patients in the highest 2 deciles. Heart transplantation may reduce survival in the least ill patients but is clearly strongly beneficial for severely ill patients, offering reductions of 20 to 35 percentage points in probability of death when compared with remaining on the waiting list or not receiving a transplant at all. CONCLUSIONS: Our analyses indicate that criteria other than severity of illness as measured by the probability of death are, in practice, dominant in the allocation of donated hearts for transplantation. High percentages of patients listed as well as those transplanted are not expected to undergo a substantial increase in probability of survival, and some are likely to be harmed. A survival benefit is anticipated only for severely ill patients. Estimation of the projected survival benefit of listing and of transplantation is feasible and may be used to prioritize patients and lead to the best use of donated organs.

Follow-Up Studies↗

PubMed2004

Beyond survival: the burden of disease in decision making in organ transplantation.

Organ transplantation has long been perceived as a life-saving intervention. However, it is now recognized that the broader objectives include reducing the patient's burden of disease. The components of the burden of disease include (i) mortality, (ii) morbidity, (iii) disability, (iv) psychological distress and (v) resource use. These components may be correlated either positively or negatively, reflecting common disease antecedents or the trade-offs in clinical decisions. Our proposed approach to modeling includes measures of each outcome and, explicitly, their correlations. Its results are the predicted independent and joint probabilities that a patient will experience any specified range of each measure and of combinations of the measures. The methods were tested with data on mortality, morbidity and resource use from patients following kidney transplantation. The predicted probabilities of the measures are consistent with their observed values and distinguish among the prognoses of patients with distinctive risks, such as diabetics. We have shown, and we believe, that it is feasible to assess the interrelated components of the burden of disease in individual patients and hope that our approach may serve as a starting point in the development of a program for inclusion of alternative measures of outcome in decision making in organ transplantation.

Cost of Illness↗

PubMed2003

Quantum Monte Carlo method using phase-free random walks with slater determinants.

We develop a quantum Monte Carlo method for many fermions using random walks in the space of Slater determinants. An approximate approach is formulated with a trial wave function |Psi(T)> to control the phase problem. Using a plane-wave basis and nonlocal pseudopotentials, we apply the method to Be, Si, and P atoms and dimers, and to bulk Si supercells. Single-determinant wave functions from density functional theory calculations were used as |Psi(T)> with no additional optimization. The calculated binding energies of dimers and cohesive energy of bulk Si are in excellent agreement with experiments and are comparable to the best existing theoretical results.

Journal Article↗

PubMed2003

Time-to-event modeling of competing risks with intervening states in transplantation.

The criteria for the selection of who among the persons on the waiting is to receive an organ that has become available and who is to be placed on the list to begin with are the most contentious issues in organ transplantation. The decisions of whom to list and whom to transplant should take into account the net benefit to the individual patient and to the affected group as a whole. We present a method to compute the survival benefit by means of fully parametric modeling of the competing events (transplantation, death while awaiting the transplant, removal for other reasons), taking into account the transplant as an intervening state on the path to death post-transplant, and apply it to decisions whether to list or not list and whether to transplant or to leave on the waiting list or to remove from the list. The data were obtained from the Organ Procurement and Transplantation Network. They describe the outcomes of listings in January, 1996 through June, 1999, with a follow up of at least 1 year possible for all cases. The models produce estimates of event probabilities that accord well with the observed probabilities and predictions of the survival benefit due to transplantation that range from small negative values to increases in survival probability of 20-40% points in liver and heart transplantation, with the larger benefits generally seen in the more severely ill transplant candidates. These estimates are stable under variations of case mix, as ascertained by bootstrap analysis. The survival benefit of alternative actions can be calculated for the complex circumstances encountered clinically - competing sequential events whose probability evolves over time. The range and stability of the estimates are sufficient to permit the use of this measure to rank candidates for listing and for transplantation.

Decision Support Techniques↗