Solid state effects on the electronic structure of H2OEP.

We present the results of a joint experimental and theoretical investigation concerning the effect of crystal packing on the electronic properties of the H2OEP molecule. Thin films, deposited in ultra high vacuum on metal surfaces, are investigated by combining valence band photoemission, inverse photoemission, and X-ray absorption spectroscopy. The spectra of the films are compared, when possible, with those measured in the gas phase. Once many-body effects are included in the calculations through the GW method, the electronic structure of H2OEP in the film and gas phase are accurately reproduced for both valence and conduction states. Upon going from an isolated molecule to the film phase, the electronic gap shrinks significantly and the lowest unoccupied molecular orbital (LUMO) and LUMO + 1 degeneracy is removed. The calculations show that the reduction of the transport gap in the film is entirely addressable to the enhancement of the electronic screening.

Approximate treatment of semicore states in GW calculations with application to Au clusters.

We address the treatment of transition metal atoms in GW electronic-structure calculations within the plane-wave pseudo-potential formalism. The contributions of s and p semi-core electrons to the self-energy, which are essential to grant an acceptable accuracy, are dealt with using a recently proposed scheme whereby the exchange components are treated exactly at the G0W0 level, whereas a suitable approximation to the correlation components is devised. This scheme is benchmarked for small gold nano-clusters, resulting in ionization potentials, electron affinities, and density of states in very good agreement with those obtained from calculations where s and p semicore states are treated as valence orbitals, and allowing us to apply this same scheme to clusters of intermediate size, Au20 and Au32, that would be otherwise very difficult to deal with.

Energy-level alignment in organic dye-sensitized TiO2 from GW calculations.

The electronic energy levels of some representative isolated and oxide-supported organic dyes, relevant for photovoltaic applications, are investigated using many-body perturbation theory within the GW approximation. We consider a set of all-organic dyes (denominated L0, L2, L3, and L4) featuring the same donor and anchor groups and differing for the linker moieties. We first calculate the energy levels of the isolated molecules, thus allowing us to address the effects of the different linker groups, and resulting in good agreement with photo-electron spectroscopic and electrochemical data. We then consider the L0 dye adsorbed on the (101) surface of anatase-TiO2. We find a density of occupied states in agreement with experimental photo-electron data. The HOMO-LUMO energy gap of the L0 dye is found to be reduced by ~1 eV upon adsorption. Our results validate the reliability of GW calculations for photovoltaic applications and point to their potential as a powerful tool for the screening and rational design of new components of electrochemical solar cells.

Importance of semicore states in GW calculations for simulating accurately the photoemission spectra of metal phthalocyanine molecules.

The quasi-particle energy levels of the Zn-Phthalocyanine (ZnPc) molecule calculated with the GW approximation are shown to depend sensitively on the explicit description of the metal-center semicore states. We find that the calculated GW energy levels are in good agreement with the measured experimental photoemission spectra only when explicitly including the Zn 3s and 3p semicore states in the valence. The main origin of this effect is traced back to the exchange term in the self-energy GW approximation. Based on this finding, we propose a simplified approach for correcting GW calculations of metal phthalocyanine molecules that avoids the time-consuming explicit treatment of the metal semicore states. Our method allows for speeding up the calculations without compromising the accuracy of the computed spectra.

Communication: electronic band gaps of semiconducting zig-zag carbon nanotubes from many-body perturbation theory calculations.

Electronic band gaps for optically allowed transitions are calculated for a series of semiconducting single-walled zig-zag carbon nanotubes of increasing diameter within the many-body perturbation theory GW method. The dependence of the evaluated gaps with respect to tube diameters is then compared with those found from previous experimental data for optical gaps combined with theoretical estimations of exciton binding energies. We find that our GW gaps confirm the behavior inferred from experiment. The relationship between the electronic gap and the diameter extrapolated from the GW values is also in excellent agreement with a direct measurement recently performed through scanning tunneling spectroscopy.

Training novice robot surgeons: Proctoring provides same results as simulator-generated guidance.

To understand the influence of proctored guidance versus simulator generated guidance (SGG) on the acquisition dexterity skills in novice surgeons learning RAS (robot assisted surgery). Prospective non-blinded 3-arm randomised controlled trial (RTC). Exclusion criteria: previous experience in RAS or robotic surgery simulation. The participants were assigned to three different intervention groups and received a different form of guidance: (1) proctored guidance, (2) simulator generated guidance, (3) no guidance, during training on virtual reality (VR) simulator. All participants were asked to complete multiple questionnaires. The training was the same in all groups with the exception of the intervention part. Catharina Hospital Eindhoven, The Netherlands. A total of 70 Dutch medical students, PhD-students, and surgical residents were included in the study. The participants were randomly assigned to one of the three groups. Overall, all the participants showed a significant improvement in their dexterity skills after the training. There was no significant difference in the improvement of surgical skills between the three different intervention groups. The proctored guidance group reported a higher participant satisfaction compared to the simulator-generated guidance group, which could indicate a higher motivation to continue the training. This study showed that novice surgeons. Significantly increase their dexterity skills in RAS after a short time of practicing on simulator. The lack of difference in results between the intervention groups could indicate there is a limited impact of "human proctoring" on dexterity skills during surgical simulation training. Since there is no difference between the intervention groups the exposure alone of novice surgeons to the robotic surgery simulator could possibly be sufficient to achieve a significant improvement of dexterity skills during the initial steps of RAS learning.

Structured robot-assisted surgery training curriculum for residents in Urology and impact on future surgical activity.

To gain insight into the availability of training for robot assisted surgery (RAS) and the possibility to perform RAS during Dutch residency curriculum and to analyze the effects on surgical skills by the introduction of an advanced course in RAS for residents. A combination of a validated snap shot survey and a prospective cohort study. Structured advanced RAS training including virtual reality (VR) simulation, dry and wet lab facility at ORSI academy (Belgium). A snap-shot survey has been sent to all the residents and specialists in Urology graduated during the years 2017-2020 in Netherlands. Among residents, only last year residents (5th and 6th year) have been considered for the RAS training. Although most of the residents (88.2%) and young urologists (95%) were asked to follow a basic training or meet basic requirements before starting RAS, the requirements set by the educators were different from center to center. Some of them were required to attend only an online course on RAS, whereas others were asked to achieve threshold scores at VR simulator and participate in a standardized course at a training institute. The attendance to a structured advanced course in RAS showed a significant increase in surgical skills. Our study shows residents in urology are allowed to perform RAS during their residency though the criteria for starting RAS differ significantly amongst the teaching hospitals. To guarantee a basic level of skills and knowledge a structured, (multi-step) training and certification program for RAS should be implemented.

Valence electronic properties of porphyrin derivatives.

We present a combined experimental and theoretical investigation of the valence electronic structure of porphyrin-derived molecules. The valence photoemission spectra of the free-base tetraphenylporphyrin and of the octaethylporphyrin molecule were measured using synchrotron radiation and compared with theoretical spectra calculated using the GW method and the density-functional method within the generalized gradient approximation. Only the GW results could reproduce the experimental data. We found that the contribution to the orbital energies due to electronic correlations has the same linear behavior in both molecules, with larger deviations in the vicinity of the HOMO level. This shows the importance of adequate treatment of electronic correlations in these organic systems.

Linear and nonlinear susceptibilities from diffusion quantum Monte Carlo: application to periodic hydrogen chains.

We calculate the linear and nonlinear susceptibilities of periodic longitudinal chains of hydrogen dimers with different bond-length alternations using a diffusion quantum Monte Carlo approach. These quantities are derived from the changes in electronic polarization as a function of applied finite electric field--an approach we recently introduced and made possible by the use of a Berry-phase, many-body electric-enthalpy functional. Calculated susceptibilities and hypersusceptibilities are found to be in excellent agreement with the best estimates available from quantum chemistry--usually extrapolations to the infinite-chain limit of calculations for chains of finite length. It is found that while exchange effects dominate the proper description of the susceptibilities, second hypersusceptibilities are greatly affected by electronic correlations. We also assess how different approximations to the nodal surface of the many-body wave function affect the accuracy of the calculated susceptibilities.

Advanced capabilities for materials modelling with Quantum ESPRESSO.

Quantum EXPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the-art electronic-structure techniques, based on density-functional theory, density-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudopotential and projector-augmented-wave approaches. Quantum EXPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement their ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

Fraction of boroxol rings in vitreous boron oxide from a first-principles analysis of Raman and NMR spectra.

We determine the fraction f of B atoms belonging to boroxol rings in vitreous boron oxide through a first-principles analysis. After generating a model structure of vitreous B2O3 by first-principles molecular dynamics, we address a large set of properties, including the neutron structure factor, the neutron density of vibrational states, the infrared spectra, the Raman spectra, and the 11B NMR spectra, and find overall good agreement with corresponding experimental data. From the analysis of Raman and 11B NMR spectra, we yield consistently for both probes a fraction f of approximately 0.75. This result indicates that the structure of vitreous boron oxide is largely dominated by boroxol rings.

Medium-range structural properties of vitreous germania obtained through first-principles analysis of vibrational spectra.

We analyze the principal vibrational spectra of vitreous GeO(2) and derive therefrom structural properties referring to length scales beyond the basic tetrahedral unit. We generate a model structure that yields a neutron structure factor in accord with experiment. The inelastic-neutron, the infrared, and the Raman spectra, calculated within a density-functional approach, also agree with respective experimental spectra. The accord for the Raman spectrum supports a Ge-O-Ge angle distribution centered at 135 degrees. The Raman feature X(2) is found to result from vibrations in three-membered rings, and therefore constitutes a distinctive characteristic of the medium-range structure.

Dielectric response of periodic systems from quantum Monte Carlo calculations.

We present a novel approach that allows us 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 wave function, 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.

Ab initio molecular dynamics in a finite homogeneous electric field.

We treat homogeneous electric fields within density functional calculations with periodic boundary conditions. A nonlocal energy functional depending on the applied field is used within an ab initio molecular dynamics scheme. The reliability of the method is demonstrated in the case of bulk MgO for the Born effective charges, and the high- and low-frequency dielectric constants. We evaluate the static dielectric constant by performing a damped molecular dynamics in an electric field and avoiding the calculation of the dynamical matrix. Application of this method to vitreous silica shows good agreement with experiment and illustrates its potential for systems of large size.

Concentration of small ring structures in vitreous silica from a first-principles analysis of the Raman spectrum.

Using a first-principles approach, we calculate Raman spectra for a model structure of vitreous silica. We develop a perturbational method for calculating the dielectric tensor in an ultrasoft pseudopotential scheme and obtain Raman coupling tensors by finite differences with respect to atomic displacements. For frequencies below 1000 cm(-1), the parallel-polarized Raman spectrum of vitreous silica is dominated by oxygen bending motions, showing a strong sensitivity to the intermediate range structure. By modeling the Raman coupling, we derive estimates for the concentrations of three- and four-membered rings from the experimental intensities of the Raman defect lines.