268 research outputs found

### Dielectric Response of Periodic Systems from Quantum Monte Carlo Calculations

We present a novel approach that allows to calculate the dielectric response
of periodic systems in the quantum Monte Carlo formalism. We employ a many-body
generalization for the electric enthalpy functional, where the coupling with
the field is expressed via the Berry-phase formulation for the macroscopic
polarization. A self-consistent local Hamiltonian then determines the
ground-state wavefunction, allowing for accurate diffusion quantum Monte Carlo
calculations where the polarization's fixed point is estimated from the average
on an iterative sequence, sampled via forward-walking. This approach has been
validated for the case of an isolated hydrogen atom, and then applied to a
periodic system, to calculate the dielectric susceptibility of
molecular-hydrogen chains. The results found are in excellent agreement with
the best estimates obtained from the extrapolation of quantum-chemistry
calculations.Comment: 5 page 2figure

### GW quasi-particle spectra from occupied states only

We introduce a method that allows for the calculation of quasi-particle
spectra in the GW approximation, yet avoiding any explicit reference to empty
one-electron states. This is achieved by expressing the irreducible
polarizability operator and the self-energy operator through a set of linear
response equations, which are solved using a Lanczos-chain algorithm. We first
validate our approach by calculating the vertical ionization energies of the
benzene molecule and then show its potential by addressing the spectrum of a
large molecule such as free-base tetraphenylporphyrin.Comment: 4 pages, 3 figure

### Signature of small rings in the Raman spectra of normal and compressed amorphous silica: A combined classical and ab initio study

We calculate the parallel (VV) and perpendicular (VH) polarized Raman spectra
of amorphous silica. Model SiO2 glasses, uncompressed and compressed, were
generated by a combination of classical and ab initio molecular-dynamics
simulations and their dynamical matrices were computed within the framework of
the density functional theory. The Raman scattering intensities were determined
using the bond-polarizability model and a good agreement with experimental
spectra was found. We confirm that the modes associated to the fourfold and
threefold rings produce most of the Raman intensity of the D1 and D2 peaks,
respectively, in the VV Raman spectra. Modifications of the Raman spectra upon
compression are found to be in agreement with experimental data. We show that
the modes associated to the fourfold rings still exist upon compression but do
not produce a strong Raman intensity, whereas the ones associated to the
threefold rings do. This result strongly suggests that the area under the D1
and D2 peaks is not directly proportional to the concentration of small rings
in amorphous SiO2.Comment: 21 pages, 8 figures. Phys. Rev. B, in pres

### Pengaruh Bimbingan Kelompok terhadap Peningkatan Empati Siswa di Kelas X Mipa 3 SMA Negeri 12 Pekanbaru Tahun Ajaran 2015/2016

This study aims to determine the effect of the increase in emaptistudents in the experimental group. The research method Control Group pre-test-posttest.Subjek in this study were 22 students who have low empathy in class X SMA Negeri12 Pekanbaru.Hasil MIPA research is to test different before and after the implementationof group counseling in the experimental group using the program SPSS formula obtainedresults Wilcoxon Z = -3125 with sig. (2-tailed) of 0002, because sig. (2-tailed) <0.05 then"There are differences in the level of empathy of students before and after the groupcounseling was held in the experimental group". To test the difference in pre-test and posttestin the control group using SPSS formula obtained results Wilcoxon Z = -2679 sig. (2-tailed) 0789 because sig. (2-tailed) <0.05 then "There is no difference in the level ofempathy of students pre-test and post-test in the control group". Then to see the differencesin post test post test experimental group with the control group used formula MannWhitney in SPSS with the result Z = -3983 with results sig. (2-tailed) of 0.000. Karna sig.(2-tailed) 0.000 <0.05 then it can be concluded "there are significant differences betweenthe students empathy post test post test experimental group with the control group" whilethe contribution of group guidance menibngkatkan empathy in students by 55

### Motivasidanhasilstudisiswamiskinsmpnegeri3kecamatan Pujud

Motivation and study result of SMPN 3 PUJUD's poor students: Budi Pramaharja, 1205188179, 2014. Study program: guidance and counseling; science department, Education and science faculty: Riau University. Counselor I: Prof. Dr. Zulfan Saam, M. Si; Counselor II: Dra. Tri Umari, M. Si.This research aims to know the illustration of poor student' study motivation in SMPN 3 PUJUD that back grounded by some reason that seen in school such as : the problem about low motivation and the study result of poor student in SMPN 3 PUJUD.The method that be used in this research is Descriptive method in percentage, the location of research is in SMPN 3 PUJUD. This research subject is VII, VIII, IX students' class. Giving the study motivation is taken by sample technique or saturate sample. Because the sum of population in this research is small. Data and the instrument of data collection is questionnaire form about motivation and study of result poor student in SMPN 3 PUJUD that done by researcher and be guided at lattice work with the alternative of multiple choices: frequent, often, sometimes, seldom and never. Data analysis technique uses the percentage formula (Anas Sudijono, 2004:170)Based on the research and data processing that had been done, can be resumed : (1) based on the result of data processing can be know the general descriptive of study motivation: VII, VIII, IX students' class of SMPN 3 PUJUD, PUJUD district, school year 2013-2014, that is by paying attention the intensity of giving the poor students' study motivation, with the result that low level category. The poor students' study motivation is be categorized low level, be market by : a) have not the lesson book, b) less the parent's attention, c) seldom visit the library, d) less the media in giving the assist to the development of potential students in studying, so that become the adult human and responsible. (2) the description of poor students' study result in SMPN 3 PUJUD can be know that is the students of VII, VIII, IX class, school year 2013-2014, low level dominantly . the low subject is under KKM on Pendidikan Pancasila dan Kewarganegaraan (citizen) lesson by rating 47,5 before remedial, and on IPA (science) lesson by rating 45 before remedial. (3) constraints that be faced by the poor students like as helping parent, long distance from house to school, this case result the low study motivation and study result for students

### Pengaruh Konseling Kelompok terhadap Pengurangan Stres Akademik Siswa Kelas X Sman 2 Siakhulu T.p 2014/2015

The title of this research is â€˜' Effect of Group Counseling towards the Reduction of Academic Stress of First â€“ Year Students of Academic Year 2014/2015 at SMAN 2 Siak Hulu''. The purposes of this research were 1) to know the description of students' academic stress before given group counseling service, 2) to know the dynamic description of the group counseling implementation in order to reduce students' academic stress, 3) to know the description of students' academic stress reduction after given group counseling service, 4) to know how much the difference of students' academic stress reduction before and after given group counseling service is, 5) to know how much the influence of group counseling towards the reduction of students' academic stress is. The instrument used in this research was academic stress scale. The subjects of this research were 101 first â€“ year students of academic year 2014/2015 at SMAN 2 Siak Hulu. The sample of this research was 30 students and they were selected by simple random sampling. The description of academic stress before given group counseling was in high category (50%), medium category (46,67%), and low category (3,33%). The description of students' academic stress reduction after given group counseling was in high category (10%), medium category (53,33%), and low category (36,67%). Based on the result of the calculation of the correlation coefficient between X1 and X2 was 0,505 and the determination coefficient (r2) was 0,255. This means that the contribution of group counseling towards the reduction of academic stress was 25,5%. Therefore, it can be known that ttest was bigger than ttable, (7,93 > 2,000) so that Ha was accepted . So, it means that there was influence of group counseling towards the reduction of academic stress of first â€“ year students of academic year 2014/2015 at SMAN 2 Siak Hulu

### Photoelasticity of crystalline and amorphous silica from first principles

Based on density-functional perturbation theory we have computed from first
principles the photoelastic tensor of few crystalline phases of silica at
normal conditions and high pressure (quartz, $\alpha$-cristobalite,
$\beta$-cristobalite) and of models of amorphous silica (containig up to 162
atoms), obtained by quenching from the melt in combined classical and
Car-Parrinello molecular dynamics simulations. The computational framework has
also been checked on the photoelastic tensor of crystalline silicon and MgO as
prototypes of covalent and ionic systems. The agreement with available
experimental data is good.
A phenomenological model suitable to describe the photoelastic properties of
different silica polymorphs is devised by fitting on the ab-initio data.Comment: ten figure

### Artificial photosynthesis: photoanodes based on polyquinoid dyes onto mesoporous tin oxide surface

Dye-sensitized photoelectrochemical cells represent an appealing solution for artificial photosynthesis, aimed at the conversion of solar light into fuels or commodity chemicals. Extensive efforts have been directed towards the development of photoelectrodes combining semiconductor materials and organic dyes; the use of molecular components allows to tune the absorption and redox properties of the material. Recently, we have reported the use of a class of pentacyclic quinoid organic dyes (KuQuinone) chemisorbed onto semiconducting tin oxide as photoanodes for water oxidation. In this work, we investigate the effect of the SnO2 semiconductor thickness and morphology and of the dye-anchoring group on the photoelectrochemical performance of the electrodes. The optimized materials are mesoporous SnO2 layers with 2.5 mu m film thickness combined with a KuQuinone dye with a 3-carboxylpropyl-anchoring chain: these electrodes achieve light-harvesting efficiency of 93% at the maximum absorption wavelength of 533 nm, and photocurrent density J up to 350 mu A/cm(2) in the photoelectrochemical oxidation of ascorbate, although with a limited incident photon-to-current efficiency of 0.075%. Calculations based on the density functional theory (DFT) support the role of the reduced species of the KuQuinone dye via a proton-coupled electron transfer as the competent species involved in the electron transfer to the tin oxide semiconductor. Finally, a preliminary investigation of the photoelectrodes towards benzyl alcohol oxidation is presented, achieving photocurrent density up to 90 mu A/cm(2) in acetonitrile in the presence of N-hydroxysuccinimide and pyridine as redox mediator and base, respectively. These results support the possibility of using molecular-based materials in synthetic photoelectrochemistry.[GRAPHICS]

### Resonant Lifetime of Core-Excited Organic Adsorbates from First Principles

We investigate by first-principles simulations the resonant electron-transfer
lifetime from the excited state of an organic adsorbate to a semiconductor
surface, namely isonicotinic acid on rutile TiO$_2$(110). The
molecule-substrate interaction is described using density functional theory,
while the effect of a truly semi-infinite substrate is taken into account by
Green's function techniques. Excitonic effects due to the presence of
core-excited atoms in the molecule are shown to be instrumental to understand
the electron-transfer times measured using the so-called core-hole-clock
technique. In particular, for the isonicotinic acid on TiO$_2$(110), we find
that the charge injection from the LUMO is quenched since this state lies
within the substrate band gap. We compute the resonant charge-transfer times
from LUMO+1 and LUMO+2, and systematically investigate the dependence of the
elastic lifetimes of these states on the alignment among adsorbate and
substrate states.Comment: 24 pages, 6 figures, to appear in Journal of Physical Chemistry

- â€¦