5,633 research outputs found
Water hexamer: Self-consistent phonons versus reversible scaling versus replica exchange molecular dynamics
Classical free energies for the cage and prism isomers of water hexamer
computed by the self- consistent phonons (SCP) method and reversible scaling
(RS) method are presented for several flexible water potentials. Both methods
have been augmented with a rotational correction for improved accuracy when
working with clusters. Comparison of the SCP results with the RS results
suggests a fairly broad temperature range over which the SCP approximation can
be expected to give accurate results for systems of water clusters, and
complements a previously reported assessment of SCP. Discrepancies between the
SCP and RS results presented here, and recently published replica exchange
molecular dynamics (REMD) results bring into question the convergence of the
REMD and accompanying replica exchange path integral molecular dynamics
results. In addition to the ever-present specter of unconverged results,
several possible sources for the discrepancy are explored based on inherent
characteristics of the methods used.Comment: Submitted to Journal Chemical Physic
Anomalous quantum and isotope effects in water clusters: Physical phenomenon, model artifact, or bad approximation?
Free energy differences are computed for
several isomers of water hexamer relative to the "prism" isomer using the
self-consistent phonons method. % We consider the
isotope effect defined by the quantity , and the quantum effect, , and evaluate them using different flexible
water models. While both and are found
to be rather small for all of the potentials, they are especially small for two
of the empirical models, q-TIP4P/F and TTM3-F, compared to q-SPC/Fw and the two
{\it abinitio}-based models, WHBB and HBB2-pol. This qualitative difference in
the properties of different water models cannot be explained by one being "more
accurate" than the other. We speculate as to whether the observed anomalies are
caused by the special properties of water systems, or are an artifact of either
the potential energy surface form/parametrization or the numerical
approximation used.Comment: Submitted to Journal of Chemical Physic
Assessing the Performance of the Diffusion Monte Carlo Method as Applied to the Water Monomer, Dimer, and Hexamer
The Diffusion Monte Carlo (DMC) method is applied to the water monomer,
dimer, and hexamer, using q-TIP4P/F, one of the most simple, empirical water
models with flexible monomers. The bias in the time step () and
population size () is investigated. For the binding energies, the bias in
cancels nearly completely, while a noticeable bias in still
remains. However, for the isotope shift, (e.g, in the dimer binding energies
between (HO) and (DO)) the systematic errors in do
cancel. Consequently, very accurate results for the latter (within
kcal/mol) are obtained with relatively moderate numerical effort (). For the water hexamer and its (DO) isotopomer the DMC results
as a function of are examined for the cage and prism isomers. For a given
isomer, the issue of the walker population leaking out of the corresponding
basin of attraction is addressed by using appropriate geometric constraints.
The population size bias for the hexamer is more severe, and in order to
maintain accuracy similar to that of the dimer, the population size must
be increased by about two orders of magnitude. Fortunately, when the energy
difference between cage and prism is taken, the biases cancel, thereby reducing
the systematic errors to within kcal/mol when using a population of
walkers. Consequently, a very accurate result for the
isotope shift is also obtained. Notably, both the quantum and the isotope
effects for the prism-cage energy difference are small.Comment: 11 pages, 5 figures, 36 references. Submitted to the Journal of
Physical Chemistr
Small size boundary effects on two-pion interferometry
The Bose-Einstein correlations of two identically charged pions are derived
when these particles, the most abundantly produced in relativistic heavy ion
collisions, are confined in finite volumes. Boundary effects on single pion
spectrum are also studied. Numerical results emphasize that conventional
formulation usually adopted to describe two-pion interferometry should not be
used when the source size is small, since this is the most sensitive case to
boundary effects. Specific examples are considered for better illustration.Comment: more discussion on Figure4 and diffuse boundar
Policy Recommendations for Meeting the Grand Challenge to Eradicate Social Isolation
This brief was created forSocial Innovation for America’s Renewal, a policy conference organized by the Center for Social Development in collaboration with the American Academy of Social Work & Social Welfare, which is leading theGrand Challenges for Social Work initiative to champion social progress. The conference site includes links to speeches, presentations, and a full list of the policy briefs
Development of the ATAQ-IPF: a tool to assess quality of life in IPF
<p>Abstract</p> <p>Background</p> <p>There is no disease-specific instrument to assess health-related quality of life (HRQL) in patients with idiopathic pulmonary fibrosis (IPF).</p> <p>Methods</p> <p>Patients' perspectives were collected to develop domains and items for an IPF-specific HRQL instrument. We used item variance and Rasch analysis to construct the ATAQ-IPF (A Tool to Assess Quality of life in IPF).</p> <p>Results</p> <p>The ATAQ-IPF version 1 is composed of 74 items comprising 13 domains. All items fit the Rasch model. Domains and the total instrument possess acceptable psychometric characteristics for a multidimensional questionnaire. The pattern of correlations between ATAQ-IPF scores and physiologic variables known to be important in IPF, along with significant differences in ATAQ-IPF scores between subjects using versus those not using supplemental oxygen, support its validity.</p> <p>Conclusions</p> <p>Patient-centered and careful statistical methodologies were used to construct the ATAQ-IPF version 1, an IPF-specific HRQL instrument. Simple summation scoring is used to derive individual domain scores as well as a total score. Results support the validity of the ATAQ-IPF, and future studies will build on that validity.</p
Observations of the copulatory behaviour of the Ocean pout (Macrozoarces americanus)
The Ocean Pout (Macrozoarces americanus) is a common species of eelpout found off the coast of Newfoundland. It has previously been suggested that the Ocean Pout is an internally fertilizing species but no direct observations of this have been reported. What is though to be the first observation of the copulatory behaviour of mature Ocean pout in the wild is described here. Similar behaviour was videotaped under laboratory condition
The 1% Campaign
The purpose of The1% Campaign is to raise scholarship funds for first year VCU students who are dependents of VCU/VCUHS employees. Honoring Dr. Grace E. Harris, this fund will eventually provide tuition and fees for new students for need-based and/or merit-based scholarships. The ultimate goal of The 1% Campaign is to entirely fund one percent of the entering first year class which is approximately thirty to thirty-five students. The recipients will be named Grace E. Harris Scholars
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