158 research outputs found
A simple and efficient approach to the optimization of correlated wave functions
We present a simple and efficient method to optimize within energy
minimization the determinantal component of the many-body wave functions
commonly used in quantum Monte Carlo calculations. The approach obtains the
optimal wave function as an approximate perturbative solution of an effective
Hamiltonian iteratively constructed via Monte Carlo sampling. The effectiveness
of the method as well as its ability to substantially improve the accuracy of
quantum Monte Carlo calculations is demonstrated by optimizing a large number
of parameters for the ground state of acetone and the difficult case of the
state of hexatriene.Comment: 5 pages, 1 figur
Perturbatively selected configuration-interaction wave functions for efficient geometry optimization in quantum Monte Carlo
We investigate the performance of a class of compact and systematically
improvable Jastrow-Slater wave functions for the efficient and accurate
computation of structural properties, where the determinantal component is
expanded with a perturbatively selected configuration interaction scheme
(CIPSI). We concurrently optimize the molecular ground-state geometry and full
wave function -- Jastrow factor, orbitals, and configuration interaction
coefficients-- in variational Monte Carlo (VMC) for the prototypical case of
1,3-trans-butadiene, a small yet theoretically challenging -conjugated
system. We find that the CIPSI selection outperforms the conventional scheme of
correlating orbitals within active spaces chosen by chemical intuition: it
gives significantly better variational and diffusion Monte Carlo energies for
all but the smallest expansions, and much smoother convergence of the geometry
with the number of determinants. In particular, the optimal bond lengths and
bond-length alternation of butadiene are converged to better than one m\AA\
with just a few thousand determinants, to values very close to the
corresponding CCSD(T) results. The combination of CIPSI expansion and VMC
optimization represents an affordable tool for the determination of accurate
ground-state geometries in quantum Monte Carlo
Excited states with selected CI-QMC: chemically accurate excitation energies and geometries
We employ quantum Monte Carlo to obtain chemically accurate vertical and
adiabatic excitation energies, and equilibrium excited-state structures for the
small, yet challenging, formaldehyde and thioformaldehyde molecules. A key
ingredient is a robust protocol to obtain balanced ground- and excited-state
Jastrow-Slater wave functions at a given geometry, and to maintain such a
balanced description as we relax the structure in the excited state. We use
determinantal components generated via a selected configuration interaction
scheme which targets the same second-order perturbation energy correction for
all states of interest at different geometries, and we fully optimize all
variational parameters in the resultant Jastrow-Slater wave functions.
Importantly, the excitation energies as well as the structural parameters in
the ground and excited states are converged with very compact wave functions
comprising few thousand determinants in a minimally augmented double-
basis set. These results are obtained already at the variational Monte Carlo
level, the more accurate diffusion Monte Carlo method yielding only a small
improvement in the adiabatic excitation energies. We find that matching
Jastrow-Slater wave functions with similar variances can yield excitations
compatible with our best estimates; however, the variance-matching procedure
requires somewhat larger determinantal expansions to achieve the same accuracy,
and it is less straightforward to adapt during structural optimization in the
excited state.Comment: 11 pages, 4 figure
Low-Key Stationary and Mobile Tools for Probing the Atmospheric UHI Effect
The urban heat island (UHI) effect is created by a series of man-made surface modifications in urban areas that cause changes to the surface energy balance, resulting in higher urban surface air temperatures as compared with surrounding rural areas. Studying the UHI effect is highly amenable to hands-on undergraduate student research projects, because, among other reasons, there are low key measurement tools that allow accurate and regular stationary and mobile probing of air temperature. Here, we summarize the results of a student project at Texas A&M University that analyzed the atmospheric UHI of Bryan/College Station, a mid-size metro area in east Texas. Sling psychrometers were used for semi-regular twice daily stationary air temperature monitoring, and a low-cost electronic sensor and miniature data logger were used for mobile measurements. Stationary data from two similar, open mid-rise building locations showed typical UHI intensities of 0â2°C, while the mobile measurements identified situations with UHI intensities exceeding 6°C when traversing areas with high impervious surface fractions. Nighttime measurements showed the expected UHI intensity relations to wind speed and atmospheric pressure, while daytime data were more strongly related to urban morphology. The success of this research may encourage similar student projects that deliver baseline data to urban communities seeking to mitigate the UHI
The impact of the COVID-19 pandemic on an international rehabilitation study in MS: the CogEx experience
Pandemic restrictions have led to changes in therapy plans and disrupted rehabilitation services for people with multiple sclerosis. CogEx is an international, multicentre MS dual-intervention (cognitive rehabilitation, aerobic exercise) randomized, controlled rehabilitation trial confined to people with progressive disease. The primary outcome is cognition (processing speed).There are 11 treatment sites in six countries with participants required to make 27 site visits over 12Â weeks. Collectively, the large, in-person demands of the trial, and the varying international policies for the containment of COVID-19, might disproportionately impact the administration of CogEx. During the first lockdown, all centres closed on average for 82.9 (SD = 24.3) days. One site was required to lockdown on two further occasions. One site remained closed for 16Â months. Ten staff (19.2%) were required to quarantine and eight staff (15.4%) tested positive for COVID. 10 of 264 (3.8%) participants acquired COVID-19. All survived. The mean duration of enrollment delay has been [236.7 (SD = 214.5) days]. Restarting participants whose interventions were interrupted by the pandemic meant recalculating the intervention prescriptions for these individuals. While the impact of the pandemic on CogEx has been considerable, all study sites are again open. Participants and staff have shown considerable flexibility and resilience in keeping a complex, international endeavour running. The future in general remains uncertain in the midst of a pandemic, but there is cautious optimism the study will be completed with sufficient sample size to robustly evaluate our hypothesis and provide meaningful results to the MS community on the impact of these interventions on people with progressive MS. Trial registration: The trial was registered on September 20th 2018 at www.clinicaltrials.gov having identifier NCT03679468. Registration was performed before recruitment was initiated
The relationship between processing speed and verbal and non-verbal new learning and memory in progressive multiple sclerosis
Objective: Processing speed (PS) deficits are the most common cognitive deficits in multiple sclerosis (MS), followed by learning and memory deficits, and are often an early cognitive problem. It has been argued that impaired PS is a primary consequence of MS, which in turn decreases learning. The current analysis examined the association between PS and learning in a large cohort of individuals with progressive MS. Methods: Baseline data from a randomized clinical trial on rehabilitation taking place at 11 centers across North America and Europe were analyzed. Participants included 275 individuals with clinically definite progressive MS (primary, secondary) consented into the trial. Results: Symbol Digit Modalities Test (SDMT) significantly correlated with California Verbal Learning Test-II (CVLT-II) (r = 0.21, p = 0.0003) and Brief Visuospatial Memory TestâRevised (BVMT-R) (r = 0.516, p < 0.0001). Receiver operating characteristic (ROC) analysis of the SDMT z score to distinguish between impaired and non-impaired CVLT-II performance demonstrated an area under the curve (AUC) of 0.61 (95% confidence interval (CI): 0.55â0.68) and a threshold of â1.62. ROC analysis between SDMT and BVMT-R resulted in an AUC of 0.77 (95% CI: 0.71â0.83) and threshold of â1.75 for the SDMT z score to predict impaired BVMT-R. Conclusion: Results indicate little ability beyond chance to predict CVLT-II from SDMT (61%), albeit statistically significant. In contrast, there was a 77% chance that the model could distinguish between impaired and non-impaired BVMT-R. Several potential explanations are discussed
Evolution of Endoscopic Lesions in Steroid-Refractory Acute Severe Ulcerative Colitis Responding to Infliximab or Cyclosporine
BACKGROUND/AIMS: Few data on the evolution of endoscopic findings are available in patients with acute severe ulcerative colitis (ASUC). The aim of this study was to describe this evolution in a prospective cohort. METHODS: Patients admitted for a steroid-refractory ASUC and included in a randomized trial comparing infliximab and cyclosporine were eligible if they achieved steroid-free clinical remission at day 98. Flexible sigmoidoscopies were performed at baseline, days 7, 42 and 98. Ulcerative colitis endoscopic index of severity (UCEIS) and its sub-scores - vascular pattern, bleeding and ulceration/erosion - were post-hoc calculated. Global endoscopic remission was defined by a UCEIS of 0, and partial endoscopic remission by any UCEIS sub-score of 0. RESULTS: Among the 55 patients analyzed (29 infliximab and 26 cyclosporine), 49 (83%) had UCEIS >= 6 at baseline at baseline. Partial endoscopic remission rates were higher for bleeding than for vascular pattern and for ulcerations/erosions at day 7 (20% vs. 4% and 5% (n = 55); p CONCLUSION: In steroid-refractory ASUC patients responding to a second-line medical therapy, endoscopic remission process started with bleeding remission and was not achieved in half the patients at day 98 for vascular pattern. Infliximab provided a higher endoscopic remission rate than cyclosporine at day 98.Peer reviewe
US Cosmic Visions: New Ideas in Dark Matter 2017: Community Report
This white paper summarizes the workshop "U.S. Cosmic Visions: New Ideas in
Dark Matter" held at University of Maryland on March 23-25, 2017.Comment: 102 pages + reference
Socioeconomic Inequalities in Newborn Care During Facility and Home Deliveries: A Cross Sectional Analysis of Data from Demographic Surveillance Sites in Rural Bangladesh, India and Nepal
Background: In Bangladesh, India and Nepal, neonatal outcomes of poor infants are considerably worse than those of better-off infants. Understanding how these inequalities vary by country and place of delivery (home or facility) will allow targeting of interventions to those who need them most. We describe socio-economic inequalities in newborn care in rural areas of Bangladesh, Nepal and India for all deliveries and by place of delivery.
Methods: We used data from surveillance sites in Bangladesh, India and from Makwanpur and Dhanusha districts in Nepal, covering periods from 2001 to 2011. We used literacy (ability to read a short text) as indicator of socioeconomic status. We developed a composite score of nine newborn care practices (score range 0â9 indicating infants received no newborn care to all nine newborn care practices). We modeled the effect of literacy and place of delivery on the newborn care score and on individual practices.
Results: In all study sites (60,078 deliveries in total), use of facility delivery was higher among literate mothers. In all sites, inequalities in newborn care were observed: the difference in new born care between literate and illiterate ranged 0.35â0.80. The effect of literacy on the newborn care score reduced after adjusting for place of delivery (range score difference literate-illiterate: 0.21â0.43).
Conclusion: Socioeconomic inequalities in facility care greatly contribute to inequalities in newborn care. Improving newborn care during home deliveries and improving access to facility care are a priority for addressing inequalities in newborn care and newborn mortality
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