Computation of NMR observables: Consequences of projector-augmented wave sphere overlap.

The assumptions underlying the popular projector-augmented wave (PAW) method of computing electronic structure in solids are examined, with primary attention to the computation of NMR observables. In particular, the assumption of non-overlapping PAW spheres is studied, and it is shown that when the spheres do overlap, the errors may be significant and furthermore are of unpredictable sign. Examples are provided by comparing PAW-based calculations with and without overlapping spheres, with the highly accurate all-electron linear augmented wave method.

Relationships between elastic anisotropy and thermal expansion in A2Mo3O12 materials.

We report calculated elastic tensors, axial Grüneisen parameters, and thermal stress distributions in Al2Mo3O12, ZrMgMo3O12, Sc2Mo3O12, and Y2Mo3O12, a series of isomorphic materials for which the coefficients of thermal expansion range from low-positive to negative. Thermal stress in polycrystalline materials arises from interactions between thermal expansion and mechanical properties, and both can be highly anisotropic. Thermal expansion anisotropy was found to be correlated with elastic anisotropy: axes with negative thermal expansion were less compliant. Calculations of axial Grüneisen parameters revealed that the thermal expansion anisotropy in these materials is in part due to the Poisson effect. Models of thermal stress due to thermal expansion anisotropy in polycrystals following cooling showed thermal stresses of sufficient magnitude to cause microcracking in all cases. The thermal expansion anisotropy was found to couple to elastic anisotropy, decreasing the bulk coefficient of thermal expansion and leading to lognormal extremes of the thermal stress distributions.

Computation of Mössbauer isomer shifts from first principles.

Computation of the observables of a Mössbauer spectrum, primarily the isomer shift, from a first-principles approach is described. The framework used is density functional theory using the projector augmented wave formalism (DFT PAW), which enables efficient computation even of many-electron solids such as SnCl(2). The proper PAW version of the isomer shift is derived and shown to be correct through comparison of computed shifts and experiment in a variety of compounds based on tin, germanium and zinc. The effects of pressure are considered as well as motional effects including the Lamb-Mössbauer factor and the second-order Doppler shift.

Observable effects of mechanical stress induced by sample spinning in solid state nuclear magnetic resonance.

The stress-induced change in chemical shielding induced by sample spinning is measured and interpreted theoretically. By considering the rotating sample as an elastic body in the plane-strain approximation, the internal stress field as a function of sample size, rotation frequency, and elastic constants is determined. This stress field and the dependence of chemical shielding on strain, as determined by first-principles calculations, are combined to predict the shielding dependence on rotation frequency under isothermal conditions in single crystal gallium phosphide. The prediction is in good qualitative agreement with the experiment. Little to no effect is detected in powder samples of both gallium phosphide and copper iodide, and it is argued that this follows from the stress distribution in granular material, as opposed to bulk crystals. Finally, the temperature and pressure dependence of the chemical shielding is estimated from these considerations and found consistently to underestimate the experimental values, indicating the importance of finite-temperature anharmonic effects even in very simple solids.

Preferential binding of fluorine to aluminum in high peralkaline aluminosilicate glasses.

For two series of fluoride-containing aluminosilicate glasses of high peralkaline type, we apply 27Al, 19F, 29Si, and 23Na NMR spectroscopy to understand the structural changes introduced by the addition of alkali fluorides. Adding fluoride in concentrations above the solubility limit causes crystallization of different phases in sodium and potassium glasses despite identical composition. However, the NMR spectra reveal that the structural evolution of the precrystallized states is similar in both series. In particular, fluorine coordinates exclusively to alkaline cations and aluminum. No indication of direct bonding with silicon was found from 19F --> 29Si cross-polarization experiments. In contrast to other glass systems, double resonance experiments in these peralkaline systems show that halide addition produces at most a minor fraction of tetrahedral aluminum containing fluorine in its coordination sphere. Instead, the fluorine addition prior to crystallization converts up to about 20% of the initial tetrahedral aluminum (1 mol % in absolute units) to 5- and 6-fold coordinated aluminum. A minor portion of five-coordinated aluminum groups is considered as the intermediate to the growing fraction of octahedral aluminum in the silicate matrix. The initialization of the crystallization process is correlated with the saturation of the silicate matrix by octahedral aluminum clusters segregating out under further doping by fluoride. It is suggested that the formation of the nonframework Al-F bonds is responsible for structural relaxation, reflected by the reduction of the glass transition temperature.

A neutron scattering and nuclear magnetic resonance study of the structure of GeO2-P2O5 glasses.

Germanophosphate (GeO2-P2O5) glasses were studied with neutron diffraction, phosphorus, and oxygen nuclear magnetic resonance, calorimetry, viscosity measurements, and first-principles calculations. These data sets were combined to propose a structural model of GeO2-P2O5 glasses, which includes tetrahedrally coordinated phosphorus, formation of octahedrally coordinated germanium as P2O5 content increases, an absence of trigonally coordinated oxygen, and hence an absence of rutile-like GeO2 domains. The structural model was then used to propose explanations for both the observed composition dependence of the glass transition temperature and the fragility of the GeO2-P2O5 liquids.

Stress, strain, and NMR.

Experimental and ab initio results that demonstrate the effect of stress on the nuclear magnetic resonance spectra of materials are shown. The design of a cell that generates uniaxial compressive stress is presented, and results on gallium phosphide and lead nitrate single crystals that illustrate the observable results of the stress are shown. Tensors that relate stress and strain to changes in the chemical shielding tensors and the electric field gradient tensors are defined formally. The elements of these tensors are then computed by a density functional theory approach that makes use of planewaves and pseudopotentials. The experimental results are interpreted with the aid of the calculations. Extensions to spinning samples and to the interpretation of optical phenomena in materials are discussed.

On the spectral similarity of bridging and nonbridging oxygen in tellurites.

We show by high field (17)O solid-state nuclear magnetic resonance (NMR) and by ab initio calculations of both the NMR and the oxygen 1s photoelectron spectra that the oxygen sites in tellurite glasses show no spectroscopic distinction, even when comparing bridging and nonbridging sites. This is remarkable because two such sites differ formally by a full electronic charge, and they are readily distinguished by these same methods in silicates. We argue that this similarity arises from the symmetry breaking that occurs when the original TeO(2) crystal solid forms, due to the pseudo-Jahn-Teller distortion induced by the two additional valence electrons present in Te(IV) as compared to Si(IV).

Platinum-containing hyper-cross-linked polystyrene as a modifier-free selective catalyst for L-sorbose oxidation.

Impregnation of hyper-cross-linked polystyrene (HPS) with tetrahydrofuran (THF) or methanol (ML) solutions containing platinic acid results in the formation of Pt(II) complexes within the nanocavities of HPS. Subsequent reduction of the complexes by H2 yields stable Pt nanoparticles with a mean diameter of 1.3 nm in THF and 1.4 nm in ML. The highest selectivity (98% at 100% conversion) measured during the catalytic oxidation of L-sorbose in water is obtained with the HPS-Pt-THF complex prior to H2 reduction. During an induction period of about 100 min, L-sorbose conversion is negligible while catalytic species develop in situ. The structure of the catalyst isolated after the induction period is analyzed by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. Electron micrographs reveal a broad distribution of Pt nanoparticles, 71% of which measure less than or equal to 2.0 nm in diameter. These nanoparticles are most likely responsible for the high catalytic activity and selectivity observed. The formation of nanoparticles measuring up to 5.9 nm in diameter is attributed to the facilitated intercavity transport and aggregation of smaller nanoparticles in swollen HPS. The catalytic properties of these novel Pt nanoparticles are highly robust, remaining stable even after 15 repeated uses.

Evaluating the benefits of increasing measles immunization rates.

OBJECTIVE: To calculate the cost-effectiveness, expressed in dollars per quality-adjusted life years (QALY), of increasing measles immunization rates. DATA SOURCES/STUDY DESIGN: Published data were supplemented by expert opinion. We determined the cost savings and value of the health benefits from averting a single case of measles. Next we examined the U. S. data regarding the relationship between pre-school measles immunization and incidence rates. Finally, we calculated the cost-effectiveness of a program that would increase a locality's immunization rate to the point of disease elimination. PRINCIPAL FINDINGS: Averting a single case of measles, using "base case" assumptions, yields societal cost savings of $2,089 and an increase of 0.086 QALYs. Using a very low discount rate increases the total benefits to $2,251 in societal cost savings and 0.150 QALYs in health benefits. In general, programs to raise measles immunization rates are not cost-effective, except possibly during an outbreak of the disease or in areas with very low immunization rates. The extremely low measles incidence rates in the mid-1990s result in such programs having extremely high costs per QALY gained. CONCLUSIONS: Programs that are narrowly designed to increase immunization rates alone are not likely to be cost-effective. Yet these programs do have the potential to be cost-effective if the program design and evaluation also recognize the benefits associated with the primary and preventive care that can accompany immunizations. Such programs may also be cost-effective if they are components of a global eradication of measles.

Evaluating large and complex demonstrations: the CHAMPUS reform initiative experience. Civilian Health and Medical Program of the Uniformed Services.

OBJECTIVE: To examine the evaluation process for the CHAMPUS Reform Initiative (CRI) both to highlight issues that evaluators must consider when undertaking such projects and to provide policymakers with tools to better assess demonstration project evaluations. DATA SOURCES: The CRI evaluation. STUDY DESIGN: Case study. DATA COLLECTION: Review of CRI evaluation reports. PRINCIPAL FINDINGS: Although policymakers increasingly rely on the evaluations of demonstration projects to determine whether to extend the scope and funding of many public programs, the results of these evaluations are often difficult to assess. Despite its analytical sophistication, the CRI evaluation was no exception. The somewhat artificial time constraints imposed by policymakers made projection of the CRI's performance beyond the demonstration period particularly difficult. CONCLUSIONS: Much uncertainty generally remains even after well-planned and well-executed evaluations of demonstration projects.

HMO penetration, competition, and risk-adjusted hospital mortality.

OBJECTIVE: HMOs have been shown to have an effect on the care provided directly to their enrollees. They may also influence the care provided to individuals in fee-for-service plans through a spill-over effect. The objective of this study was to investigate the associations among HMO market penetration, HMO and hospital competition, and the quality of care received by Medicare fee-for-service patients measured by risk-adjusted hospital mortality rates. DATA SOURCES: The 1990 data for 1,927 hospitals in 134 metropolitan statistical areas (with five or more hospitals) included Medicare fee-for-service risk-adjusted mortality rates from the Medicare Hospital Information Reports, hospital characteristics from the American Hospital Association annual survey, and HMO market penetration and competition calculated from InterStudy and Group Health Association of America data. STUDY DESIGN: Statistical regression techniques were used to identify the associations between HMO market penetration, competition, and risk-adjusted mortality, controlling for other hospital characteristics and region. PRINCIPAL FINDINGS: Higher HMO market penetration and to a lesser degree increased HMO competition were associated with better mortality outcomes for fee-for-service Medicare enrollees. Competition between hospitals did not exhibit a significant association. CONCLUSIONS: HMOs may have a spill-over effect on quality of care received by individuals enrolled in fee-for-service plans. These findings may be explained by a positive effect on local practice styles or a preferential selection by HMOs for areas with better hospital care.

The effect of selective contracting on hospital costs and revenues.

OBJECTIVE: To examine the effects of selective contracting on California hospital costs and revenues over the 1983-1997 period. DATA SOURCES/STUDY SETTING: Annual disclosure data and discharge data sets for 421 California general acute care hospitals from 1980 to 1997. ANALYSIS: Using measures of competition developed from patient-level discharge data, and financial and utilization measures from the disclosure data, we estimated a fixed effect multivariate regression model of hospital costs and revenues. FINDINGS: We found that hospitals in more competitive areas had a substantially lower rate of increase in both costs and revenues over this extended period of time. For-profit hospitals lowered their costs and revenues after selective contracting was initiated relative to the cost and revenue levels of not-for-profit hospitals. The Medicare PPS has also led high-cost hospitals to lower their costs. CONCLUSIONS: The more competitive the hospital's market, the greater degree to which it has had to lower the rate of increase in costs. A similar pattern exists with regard to hospital revenues. Both of these trends appear to result from the growth of selective contracting. It remains unclear to what extent these cost reductions were the result of increased efficiency or of reduced quality. Since hospital cost growth is sensitive to the competitiveness of its market, antitrust enforcement is a critical element in any cost containment policy.

Economic analysis of two models of low-risk maternity care: a freestanding birth center compared to traditional care.

Costs were compared for two models of maternity care for low-risk pregnant women: a freestanding birth center (FSBC) and a medical model of care (MC). Sixty-nine subjects were enrolled in the FSBC group and 77 in the MC. In the FSBC group, prenatal costs were higher (mean difference $751, p

Through-bond connectivity in solids by continuous-wave spin lock.

A simple two-dimensional correlation experiment that enables determination of through-bond connectivity in the solid state is described. The experiment is performed under fast magic angle spinning (MAS) conditions. After the initial pi/2 pulse, the magnetization develops freely under the MAS Hamiltonian. The t1-period is followed by a strong spin locking pulse used as mixing period. The dipolar coupling is averaged out by magic angle spinning, and the chemical shifts and r.f.-offsets are scaled by the applied spin locking field. Hence, for strong locking conditions, the isotropic J-coupling is the dominant interaction. The mixing Hamiltonian is thus identical to the well-known TOCSY-Hamiltonian, resulting in a net through-bond magnetization transfer. The mixing-time dependence of the exchange rates is investigated. Applications to crystalline P4S7 and MgP4O11 are shown.

Is travel distance a barrier to veterans' use of VA hospitals for medical surgical care?

Lengthy travel distances may explain why relatively few veterans in the United States use VA hospitals for inpatient medical/surgical care. We used two approaches to distinguish the effect of distance on VA use from other factors such as access to alternatives and veterans' characteristics. The first approach describes how disparities in travel distance to the VA are related to other characteristics of geographic areas. The second approach involved a multivariate analysis of VA use in postal zip code areas (ZCAs). We used several sources of data to estimate the number of veterans who had priority access to the VA so that use rates could be estimated. Access to hospitals was characterized by estimated travel distance to inpatient providers that typically serve each ZCA. The results demonstrate that travel distance to the VA is variable, with veterans in rural areas traveling much farther for VA care than veterans in areas of high population density. However, Medicare recipients also travel farther in areas of low population density. In some areas veterans must travel lengthy distances for VA care because VA hospitals which were built over the past few decades are not located close to areas in which veterans reside in the 1990s. The disparities in travel distance suggest inequitable access to the VA. Use of the VA decreases with increases in travel distance only up to about 15 miles, after which use is relatively insensitive to further increases in distance. The multivariate analyses indicate that those over 65 are less sensitive to distance than younger veterans, even though those over 65 are Medicare eligible and therefore have inexpensive access to alternatives. The results suggest that proximity to a VA hospital is only one of many factors determining VA use. Further research is indicated to develop an appropriate response to the needs of the small but apparently dedicated group of VA users who are traveling very long distances to obtain VA care.

Can cost shifting continue in a price competitive environment?

Both Medicare and Medicaid are reducing payments to hospitals, and there is widespread concern that hospitals may respond by increasing prices to privately insured patients. Theoretical models of hospital behaviour have ambiguous predictions as to whether, and under what circumstances, hospitals will shift costs to private payers. This paper extends previous theoretical models and then tests empirically using data from California for the 1983-1991 period, a time of increasingly intense price competition. Hospitals did increase their prices to private payers in response to reductions in Medicare rates; they had far smaller and generally insignificant responses to changes in Medicaid reimbursement. Hospital ownership and the competitiveness of the hospital market both affected this behaviour, but there was no significant change over time. The results suggest the need to broaden our models of hospital behaviour to 'embed' them in their local markets.

Providing managed care options for a large population: evaluating the CHAMPUS reform initiative. Civilian Health and Medical Program of the Uniformed Services.

OBJECTIVES: To evaluate a managed care demonstration project in CHAMPUS (Civilian Health and Medical Program of the Uniformed Services), the insurance program covering physical and mental health care services for the dependents of active duty military personnel, military retirees, and the retirees' dependents. The demonstration project added a health maintenance organization (HMO) option and a preferred provider organization (PPO) option to the standard CHAMPUS coverage and allowed beneficiaries to select the coverage option they preferred. DATA SOURCES: Utilization, costs, access, and beneficiary satisfaction were measured using data from CHAMPUS claims records, the Defense Enrollment Eligibility Reporting System, the demonstration project contractor's HMO enrollment file, the contractor's list of network hospitals, and two surveys of CHAMPUS beneficiaries. STUDY DESIGN: Changes in utilization at 11 demonstration sites were compared with changes in utilization at 11 matched control sites. The effect of the demonstration project on costs was evaluated by estimating the costs for the demonstration sites both with and without the managed care options based on data from the control sites. Access to care and satisfaction were compared between the demonstration sites and control sites based on beneficiary surveys. DATA COLLECTION: All claims in both demonstration and control sites were used in estimating utilization changes. Two mailed surveys were sent to a randomly selected sample of active duty and retiree households with CHAMPUS beneficiaries; the sample was stratified by beneficiary type (active duty or retiree) and site. PRINCIPLE FINDINGS: Overall utilization in the CHAMPUS system decreased at the demonstration sites but stayed approximately the same at the control sites. Utilization among the enrollees in the HMO demonstration option, however, increased dramatically. Patient access to care and satisfaction generally remained at the same levels at both demonstration and control sites, but enrollees in the HMO option reported higher satisfaction. Costs to the government at the demonstration areas, based on regression estimates from the control sites, were about the same or slightly higher than what they would have been under the standard CHAMPUS system. CONCLUSIONS: Managed care plans for large government-sponsored insurance programs can reduce utilization and maintain patient access and satisfaction. Careful structuring of such plans is needed, however, if they are to reduce costs.

Anisotropy-correlated spectroscopy of quadrupolar nuclei.

The two-dimensional anisotropy-correlated NMR (2DAC) spectra of half-integer quadrupolar nuclei may be recorded by using an exchange sequence in conjunction with magic angle spinning (MAS) during evolution and detection, and off-MAS during mixing. Application of this experiment to boron oxides is described, in addition to an analysis of the spin diffusion rates in such materials.