The effect of hydrogen sulphide on ammonium bisulphite when used as an oxygen scavenger in aqueous solutions

Peer reviewedPostprin

Zero-variance zero-bias quantum Monte Carlo estimators of the spherically and system-averaged pair density

We construct improved quantum Monte Carlo estimators for the spherically- and system-averaged electron pair density (i.e. the probability density of finding two electrons separated by a relative distance u), also known as the spherically-averaged electron position intracule density I(u), using the general zero-variance zero-bias principle for observables, introduced by Assaraf and Caffarel. The calculation of I(u) is made vastly more efficient by replacing the average of the local delta-function operator by the average of a smooth non-local operator that has several orders of magnitude smaller variance. These new estimators also reduce the systematic error (or bias) of the intracule density due to the approximate trial wave function. Used in combination with the optimization of an increasing number of parameters in trial Jastrow-Slater wave functions, they allow one to obtain well converged correlated intracule densities for atoms and molecules. These ideas can be applied to calculating any pair-correlation function in classical or quantum Monte Carlo calculations.Comment: 13 pages, 9 figures, published versio

Magnetic transitions in CaMn7O12 : a Raman observation of spin-phonon couplings

The quadruple Calcium manganite (CaMn7O12) is a multiferroic material that exhibits a giant magnetically-induced ferroelectric polarization which makes it very interesting for magnetoelectric applications. Here, we report the Raman spectroscopy study on this compound of both the phonon modes and the low energy excitations from 4 K to room temperature. A detailed study of the Raman active phonon excitations shows that three phonon modes evidence a spin-phonon coupling at TN2 = 50 K. In particular, we show that the mode at 432 cm-1 associated to Mn(B)O6 (B position of the perovskite) rotations around the [111] cubic diagonal is impacted by the magnetic transition at 50 K and its coupling to the new modulation of the Mn spin in the (a,b) plane. At low energies, two large low energy excitations are observed at 25 and 47 cm-1. The first one disappears at 50 K and the second one at 90 K. We have associated these excitations to electro-magneto-active modes

Alternative separation of exchange and correlation energies in multi-configuration range-separated density-functional theory

The alternative separation of exchange and correlation energies proposed by Toulouse et al. [Theor. Chem. Acc. 114, 305 (2005)] is explored in the context of multi-configuration range-separated density-functional theory. The new decomposition of the short-range exchange-correlation energy relies on the auxiliary long-range interacting wavefunction rather than the Kohn-Sham (KS) determinant. The advantage, relative to the traditional KS decomposition, is that the wavefunction part of the energy is now computed with the regular (fully-interacting) Hamiltonian. One potential drawback is that, because of double counting, the wavefunction used to compute the energy cannot be obtained by minimizing the energy expression with respect to the wavefunction parameters. The problem is overcome by using short-range optimized effective potentials (OEPs). The resulting combination of OEP techniques with wavefunction theory has been investigated in this work, at the Hartree-Fock (HF) and multi-configuration self-consistent-field (MCSCF) levels. In the HF case, an analytical expression for the energy gradient has been derived and implemented. Calculations have been performed within the short-range local density approximation on H2, N2, Li2 and H2O. Significant improvements in binding energies are obtained with the new decomposition of the short-range energy. The importance of optimizing the short-range OEP at the MCSCF level when static correlation becomes significant has also been demonstrated for H2, using a finite-difference gradient. The implementation of the analytical gradient for MCSCF wavefunctions is currently in progress.Comment: 5 figure

Signature of the Ground-State Topology in the Low-Temperature Dynamics of Spin Glasses

We numerically address the issue of how the ground state topology is reflected in the finite temperature dynamics of the $\pm J$ Edwards-Anderson spin glass model. In this system a careful study of the ground state configurations allows to classify spins into two sets: solidary and non-solidary spins. We show that these sets quantitatively account for the dynamical heterogeneities found in the mean flipping time distribution at finite low temperatures. The results highlight the relevance of taking into account the ground state topology in the analysis of the finite temperature dynamics of spin glasses.Comment: 4 pages, 4 figures, content change

Neurotrophic factors for the treatment of Parkinson's disease

Parkinson's disease (PD) is a common neurodegenerative disorder caused by the progressive degeneration of the nigrostriatal dopaminergic pathway. The resulting loss of dopamine neurotransmission is responsible for the symptoms of the disease. Available treatments are initially successful in treating PD symptoms; however, their long-term use is associated with complications and they cannot stop the neurodegeneration. Current research aims at developing new therapies to halt/reverse the neurodegenerative process, rather than treating symptoms. Neurotrophic factors are proteins critical for maintenance and protection of neurones in the developing and adult brain. Several neurotrophic factors have been investigated for their protective effects on dopaminergic neurones. Here we review some of the most promising factors and provide an update on their status in clinical trials

The potential of neurotrophic factors for the treatment of Parkinson's disease

Parkinson’s disease (PD) is the second most common neurodegenerative disorder, with an incidence of 1.5 - 2% in the population over 60 years of age, which increases significantly with advancing age (for reviews see de Lau & Breteler, 2006; Toulouse & Sullivan, 2008). Since the Western world is experiencing significant increases in life expectancy, the incidence of PD is steadily escalating. The financial and economical burden to society of the treatment and care of PD patients is substantial and increasing. Thus, research on the causes of this debilitating disease is critical, as is the development of new treatments. PD is caused by progressive degeneration of the nigrostriatal (A9) dopaminergic pathway, which projects from the substantia nigra in the midbrain to the caudate-putamen (striatum) in the forebrain (Braak et al., 2003; Fearnley & Lees, 1991; Hoehn & Yahr, 1967; Olanow & Tatton, 1999). The resulting loss of dopamine neurotransmission in the striatum causes the cardinal symptoms of the disease: tremor at rest, rigidity and bradykinesia. One of the pathological hallmarks of PD is the appearance of intracellular protein aggregates called Lewy bodies, which are found in the substantia nigra and other brain areas (for reviews see Forno, 1996; Gibb & Lees, 1988). Lewy bodies are abnormal aggregates composed of α-synuclein, ubiquitin and other proteins. Approximately 5% of PD cases are caused by heritable genetic mutations, of which at least twelve have been identified (for review see Toulouse & Sullivan, 2008). The remaining cases are sporadic and of unknown origin, although many theories have been proposed to explain the cause of dopaminergic neuronal death which occurs in PD, such as environmental toxins, mitochondrial dysfunction with resulting oxidative stress, and inflammatory mechanisms (for reviews see Dauer & Przedborski, 2003; Dawson & Dawson, 2003; Fahn & Cohen, 1992; Long-Smith et al., 2009). At present there is no effective long-term therapy for PD. The most commonly-used treatment is administration of the dopamine precursor, levodopa, which replaces lost dopamine in the denervated striatum and relieves motor symptoms. Levodopa is generally administered in conjunction with an inhibitor of peripheral decarboxylase (carbide or benserazide), which has the effect of enhancing the central activity of levodopa and decreasing peripheral side-effects. Levodopa is successful in treating PD symptoms; however, it does not stop the ongoing neurodegeneration. Furthermore, about 50% of patients develop complications within the first five years of treatment, primarily severe motor fluctuations and dyskinesias (Freed et al., 2001; Hagell et al., 2002). Other current drug treatments include inhibitors of the dopamine breakdown enzymes catechol-O-methyl-transferase (tolcapone or entacapone) or monoamine oxidase–B (selegiline and rasagiline), and dopamine receptor agonists (bromocriptine, pergolide, pramipexole, ropinirole and others). Surgical methods involving ablation of deep brain structures such as the thalamus or pallidum, or deep brain stimulation of the subthalamic nucleus or pallidum, have also been used with good success, but these procedures are not widely-available or applicable for all patients. In summary, none of the current treatments provide safe and long-lasting relief from the symptoms and none have any effect on the progression of the disease. Much of the current research is aimed at developing new and novel therapies that will slow, halt or reverse the neurodegenerative process, rather than simply treating the symptoms of the disease. These include the use of antioxidants, anti-apoptotic agents, cell-based therapies and neuroprotective factors. Neurotrophic factors are a class of proteins that have the potential to be used as neuroprotectants in PD therapy

Bethe-Peierls Approximation for the 2D Random Ising Model

The partition function of the 2d Ising model with random nearest neighbor coupling is expressed in the dual lattice made of square plaquettes. The dual model is solved in the the mean field and in different types of Bethe-Peierls approximations, using the replica method.Comment: Plane TeX file, 21 pages, 5 figures available under request to [email protected]

Inflammation in Parkinson's disease: causes and consequences

Parkinson’s disease (PD) is the second most common progressive neurodegenerative disorder after Alzheimer’s disease (AD) with a prevalence of 0.5-1 % among persons older than 65 years of age (Toulouse & Sullivan, 2008). The incidence increases to 2.6 % in persons aged 85 and older, and has a mean age of onset of 55 years. Statistics released in 1990 from