1,399 research outputs found
How to quantify energy landscapes of solids
We explore whether the topology of energy landscapes in chemical systems
obeys any rules and what these rules are. To answer this and related questions
we use several tools: (i)Reduced energy surface and its density of states, (ii)
descriptor of structure called fingerprint function, which can be represented
as a one-dimensional function or a vector in abstract multidimensional space,
(iii) definition of a ''distance'' between two structures enabling
quantification of energy landscapes, (iv) definition of a degree of order of a
structure, and (v) definitions of the quasi-entropy quantifying structural
diversity. Our approach can be used for rationalizing large databases of
crystal structures and for tuning computational algorithms for structure
prediction. It enables quantitative and intuitive representations of energy
landscapes and reappraisal of some of the traditional chemical notions and
rules. Our analysis confirms the expectations that low-energy minima are
clustered in compact regions of configuration space ("funnels") and that
chemical systems tend to have very few funnels, sometimes only one. This
analysis can be applied to the physical properties of solids, opening new ways
of discovering structure-property relations. We quantitatively demonstrate that
crystals tend to adopt one of the few simplest structures consistent with their
chemistry, providing a thermodynamic justification of Pauling's fifth rule.Comment: Published in J. Chem. Phys. 130, 104504 (2009
Predicting the Benefits and Harms of Breast Cancer Screening: Current debates and future directions
Breast cancer is the most common cancer among women in Western countries. Presently,
women in the Netherlands have a 1 : 7 chance of developing breast cancer during
their lifetime. This means that in 2008, almost 15,000 women were newly diagnosed
with the disease. The incidence of breast cancer in the Netherlands is among the highest
in Europe.In 2006, for instance, the age-standardized incidence rate was 128 per
100,000 woman-years. As a comparison, the average in Europe was 94.3 per 100,000
woman-years. Although the probability of cure has improved over the last decennia, for
a third of all women with breast cancer, the disease will be fatal. This makes breast cancer
the most common cause of cancer death in women in Europe. The age-standardized
mortality rate in the Netherlands in 2006 was 29.8 per 100,000 woman-years, versus
26.0 per 100,000 woman-years in Europe.
Breast cancer develops as a single malignant cell with uncontrolled cell growth to a tumour
of several millimeters or centimeters in diameter. At some point in time, the tumour
may reach a size at which it becomes symptomatic. The larger the size, the less likely it
is that the tumour can be cured. It is therefore thought that by diagnosing cancer at
an earlier phase, for instance by screening, the probability of survival can be increased.
Several methods for early detection of breast cancer exist: breast self examination, examination
by a clinician or a nurse (‘clinical breast examination’), MRI or ultrasonography.
Mammography, which involves one or more X-ray images of the breasts, is considered
the best tool for examining postmenopausal women with an average risk for the disease,
because it can reach a high sensitivity (>70%) and specificity (>95%) when it is applied
on a large scale. At the same time, costs are moderate: 50 euro per screening examination
in the Netherlands.The effects of breast cancer screening using mammography were
therefore studied in several randomized controlled trials (RCTs), with various screening
ages and intervals. Screening women aged 50 and older resulted in statistically significant
reductions in breast cancer mortality, of between 25%–30% in those women that
were randomized in the screening arms of the trials. Soon after the first positive trial
outcomes, two pilot projects with large-scale mammography screening were started in
the Netherlands, which also showed substantial reductions in breast cancer mortality in
screened women. A cost-effectiveness analysis showed a favourable balance between
screening costs and potential life years gained.15 Based on these findings, mammography
screening was implemented in the Netherlands and other western countries
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