Improved modelling of liquid GeSe2_2: the impact of the exchange-correlation functional

The structural properties of liquid GeSe$_2$ are studied by using first-principles molecular dynamics in conjuncton with the Becke, Lee, Yang and Parr (BLYP) generalized gradient approximation for the exchange and correlation energy. The results on partial pair correlation functions, coordination numbers, bond angle distributions and partial structure factors are compared with available experimental data and with previous first-principle molecular dynamics results obtained within the Perdew and Wang (PW) generalized gradient approximation for the exchange and correlation energy. We found that the BLYP approach substantially improves upon the PW one in the case of the short-range properties. In particular, the Ge$-$Ge pair correlation function takes a more structured profile that includes a marked first peak due to homopolar bonds, a first maximum exhibiting a clear shoulder and a deep minimum, all these features being absent in the previous PW results. Overall, the amount of tetrahedral order is significantly increased, in spite of a larger number of Ge$-$Ge homopolar connections. Due to the smaller number of miscoordinations, diffusion coefficients obtained by the present BLYP calculation are smaller by at least one order of magnitude than in the PW case.Comment: 6 figure

Note: Accounting for pressure effects on the calculated equilibrium structure of glassy GeSe2:

First-principles molecular dynamics (FPMD) data on the structural properties of glassy GeSe2 under ambient conditions are produced by carefully considering and minimizing the effect of a residual pressure on the periodic system. When compared to previous FPMD results, this strategy leads to an improved agreement between theory and neutron diffraction experiments

Investigation of size effects on the structure of liquid GeSe2 calculated via first-principles molecular dynamics

The structural properties of liquid GeSe2 have been calculated by first-principles molecular dynamics by using a periodic simulation box containing N = 480 atoms. This has allowed a comparison with previous results obtained on a smaller system size (N = 120) [M. Micoulaut, R. Vuilleumier, and C. Massobrio, Phys. Rev. B 79, 214205 (2009)]. In the domain of first-principles molecular dynamics, we obtain an assessment of system size effects of unprecedented quality. Overall, no drastic differences are found between the two sets of results, confirming that N = 120 is a suitable size to achieve a realistic description of this prototypical disordered network. However, for N = 480, short range properties are characterized by an increase of chemical order, the number of Ge tetrahedra coordinated to four Se atoms being larger. At the intermediate range order level, size effect mostly modify the low wavevector region (k similar to 1 angstrom(-1)) in the concentration-concentration partial structure factor

A prospective, randomised, controlled clinical study on the assessment of tolerability and of clinical efficacy of Merional (hMG-IBSA) administered subcutaneously versus Merional administered intramuscularly in women undergoing multifollicular ovarian stimulation in an ART programme (IVF)

<p>Abstract</p> <p>Background</p> <p>Multifollicular ovarian stimulation (MOS) is widely used in IVF and the compliance to treatment is deeply influenced by the tolerability of the medication(s) used and by the ease of self-administration. This prospective, controlled, randomised, parallel group open label, multicenter, phase III, equivalence study has been aimed to compare the clinical effectiveness (in terms of oocytes obtained) and tolerability of subcutaneous (s.c.) self-administered versus classical intramuscular (i.m.) injections of Merional, a new highly-purified hMG preparation.</p> <p>Methods</p> <p>A total of 168 normogonadotropic women undergoing IVF were enrolled. Among them, 160 achieved pituitary suppression with a GnRH-agonist long protocol and were randomised to MOS treatment with Merional s.c. or i.m. They started MOS with a standard hMG dose between 150–300 IU, depending upon patient's age, and underwent a standard IVF procedure.</p> <p>Results</p> <p>No statistically significant difference in the mean number of collected oocytes (primary endpoint) was observed between the two study subgroups (7.46, SD 4.24 vs. 7.86, SD 4.28 in the s.c. and i.m. subgroups, respectively). As concerns the secondary outcomes, both the pregnancy and the clinical pregnancy rates were comparable between subgroups. The incidence of adverse events was similar in the two groups (2.4% vs. 3.7%, respectively). Pain at injection site was reported only the i.m. group (13.9% of patients).</p> <p>Conclusion</p> <p>Merional may be used by s.c. injections in IVF with an effectiveness in terms of retrieved oocytes that is equivalent to the one obtained with i.m administration and with a better local tolerability. With the limitations due to the sample size af this study, s.c. and i.m. administration routes seem to have the same overall safety.</p

Structural properties of glassy Ge2Se3 from first-principles molecular dynamics

The structural properties of glassy Ge2Se3 were studied in the framework of first-principles molecular dynamics by using the Becke-Lee-Yang-Parr scheme for the treatment of the exchange-correlation functional in density functional theory. Our results for the total neutron structure factor and the total pair distribution function are in very good agreement with the experimental results. When compared to the structural description obtained for liquid Ge2Se3, glassy Ge2Se3 is found to be characterized by a larger percentage of fourfold coordinated Ge atoms and a lower number of miscoordinations. However, Ge-Ge homopolar bonds inevitably occur due to the lack of Se atoms available, at this concentration, to form GeSe4 tetrahedra. Focusing on the family of glasses GexSe1-x, the present results allow a comparison to be carried out in reciprocal and real space among three prototypical glassy structures. The first was obtained at the stoichiometric composition (glassy GeSe2), the second at a Se-rich composition (glassy GeSe4) and the third at a Ge-rich composition (glassy Ge2Se3). All networks are consistent with the “8 - N” rule, in particular, glassy GeSe4, which exhibits the highest degree of chemical order. The electronic structure of glassy Ge2Se3 has been characterized by using the Wannier localized orbital formalism. The analysis of the Ge environment shows the presence of dangling, ionocovalent Ge-Se, and covalent bonds, the latter related to Ge-Ge connections. DOI: 10.1103/PhysRevB.86.224201This work was granted access by GENCI (Grand Equipement National de Calcul Intensif) under allocation 2011095071 to the HPC resources of CCRT/CINES/IDRI

The structure of liquid GeSe revisited: A first principles molecular dynamics study:

Early first-principles molecular dynamics results on liquid GeSe were characterized by shortcomings in the description of Ge-Ge (and to a lesser extent Se-Se) short range correlations. In that case the exchange-correlation functional adopted was the one devised by Perdew and Wang (PW91). In the search of improvements in the atomic-scale modelling of this liquid, we have produced new sets of data by employing two different schemes for the exchange-correlation part within the density functional theory approach. The two functionals selected are those proposed by Becke, Lee, Yang, and Parr (BLYP) and by Perdew, Burke, and Ernzerhof (PBE). The PBE results turned out to be quite similar to the PW91 ones. The BLYP results feature instead a better account of the Ge-Ge first shell of neighbors, correctly exhibiting two clear maxima separated by a deep minimum. Due to the increase in the number of the tetrahedral structural units, the atomic mobility of Ge and Se atoms in the network is reduced with respect to the PW91 case. This brings the diffusion coefficients of the two species down to values close to those of liquid Ge2Se3 and liquid GeSe2. (C) 2013 AIP Publishing LLC

Structural properties of liquid Ge₂Se₃: A first-principles study

The structural properties of liquid Ge2Se3 were investigated by first-principles molecular dynamics using the Becke-Lee-Yang-Parr scheme for the treatment of the exchange-correlation functional in density functional theory. Our data for the total neutron structure factor and the total pair-distribution function are in excellent agreement with the experimental results. The structure is made predominantly (similar to 61%) from units comprising fourfold coordinated Ge atoms in the form of Ge-GeSe3 or Ge-Se-4 motifs, but there is also a large variety of motifs in which Ge and Se are not fourfold and twofold coordinated, respectively. The miscoordinated atoms and homopolar bonds lead to a highly perturbed tetrahedral network, as reflected by diffusion coefficients that are larger than in the case of liquid GeSe2. The network does, nevertheless, exhibit intermediate range order which is associated with the Ge-Ge correlations and which manifests itself by a first sharp diffraction peak in the total neutron structure factor. The evolution of the properties of Ge-x Se1-x liquids (0 <= x <= 1) with composition is discussed.This work was granted access by GENCI (Grand Equipement National de Calcul Intensif) under allocation 2011095071 to the HPC resources of CCRT/CINES/IDRIS

Exohedral M-C-60 and M-2-C-60 (M = Pt, Pd) systems as tunable-gap building blocks for nanoarchitecture and nanocatalysis

Transition metal-fullerenes complexes with metal atoms bound on the external surface of C-60 are promising building blocks for next-generation fuel cells and catalysts. Yet, at variance with endohedral M@C-60, they have received a limited attention. By resorting to first principles simulations, we elucidate structural and electronic properties for the Pd-C-60, Pt-C-60, PtPd-C-60, Pd-2-C-60, and Pt-2-C-60 complexes. The most stable structures feature the metal atom located above a high electron density site, namely, the pi bond between two adjacent hexagons (pi-66 bond). When two metal atoms are added, the most stable configuration is those in which metal atoms still stand on p-66 bonds but tends to clusterize. The electronic structure, rationalized in terms of localized Wannier functions, provides a clear picture of the underlying interactions responsible for the stability or instability of the complexes, showing a strict relationship between structure and electronic gap

Structure, topology, rings, and vibrational and electronic properties of GexSe1-x glasses across the rigidity transition: A numerical study:

The structural, electronic, and vibrational properties of glassy GexSe1-x are studied using density-functional-based molecular dynamics. The focus is on four compositions (x = 10%, 20%, 25%, 33%) spanning the rigidity transitions and representing typical compositions of flexible, intermediate, and stressed rigid systems. We investigate structural properties including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions and compare our results with experimental findings, when available. Most noticeable is the excellent agreement found in the reproduction of the structure in real and reciprocal space which allows tracking the effect of Ge composition on the structure. Ring statistics and ring correlations are examined and followed across the rigidity transition, and the details of typical small rings show a much more complex picture than established previously. A comparison is made with simple bond models and their validity is discussed. Topological constraint analysis is performed and shows that the onset of rigidity changes substantially the angular motion inside the Ge tetrahedra, which displays increased soft bending motions in the stressed rigid phase. We then investigate the vibrational properties via the vibrational density of states and the dielectric function (infrared absorption), and discuss them with respect to experimental findings. Finally, the electronic properties are computed and show an excellent agreement with respect to previous first-principles simulations and to experiments