1,721,022 research outputs found
Ground state of a spin-crossover molecule calculated by diffusion Monte Carlo
Full text linkSpin-crossover molecules have recently emerged as a family of compounds potentially useful for implementing molecular spintronics devices. The calculations of the electronic properties of such molecules is a formidable theoretical challenge, as one has to describe the spin ground state of a transition metal as the ligand field changes. The problem is dominated by the interplay between strong electron correlation at the transition-metal site and charge delocalization over the ligands, and thus it fits into a class of problems where density functional theory may be inadequate. Furthermore, the crossover activity is extremely sensitive to environmental conditions, which are difficult to fully characterize. Here we discuss the phase transition of a prototypical spin-crossover molecule as obtained with diffusion Monte Carlo simulations. We demonstrate that the ground state changes depending on whether the molecule is in the gas or in the solid phase. As our calculation provides a solid benchmark for the theory, we then assess the performances of density functional theory. We find that the low-spin state is always overstabilized, not only by the (semi-)local functionals, but even by the most commonly used hybrids (such as B3LYP and PBE0). We then propose that reliable results can be obtained by using hybrid functionals containing about 50% of exact exchange
Predicting d0 magnetism: Self-interaction correction scheme
No full textPredicting magnetism originating from 2p orbitals is a delicate problem, which depends on the subtle interplay between covalency and Hund's coupling. Calculations based on density-functional theory and the local spin-density approximation fail in two remarkably different ways. On one hand the excessive delocalization of spin-polarized holes leads to half-metallic ground states and the expectation of room-temperature ferromagnetism. On the other hand, in some cases a magnetic ground state may not be predicted at all. We demonstrate that a simple self-interaction correction scheme modifies both these situations via an enhanced localization of the holes responsible for the magnetism and possibly Jahn-Teller distortion. In both cases the ground state becomes insulating and the magnetic coupling between the impurities becomes weak
Electronic structure of metal quinoline molecules from G0W0 calculations
No full textThe photoemission spectra of four different metal quinoline molecules, namely, the prototypical tris(8-hydroxyquinolinato)-aluminum(III) (Alq(3)) complex and the related compounds Gaq(3), Inq(3), and tris-(9-hydroxyphenalen-1oate)-aluminum(III) [Al(OP)(3)] are compared to the electronic structure computed with different first-principles methods. In general, we found that, for Alq(3), Gaq(3), and Inq(3), the molecular orbitals obtained with density functional theory and hybrid functionals represent a good approximation to the quasiparticle states. The same conclusion can be partially extended to the interesting case of Al(OP)(3), although a direct comparison between theoretical and experimental results appears rather difficult for states, which are lower in energy than the first ten highest occupied molecular orbitals. Taking our results as a starting point we critically discuss the different available experimental data concerning the charge transport gap of Alq(3)
MgN: A possible material for spintronic applications
No full textWe present rocksalt MgN as a d(0) magnet with potential for spintronics applications. Our density-functional theory calculations demonstrate that rocksalt MgN is at the verge of half-metallicity, with an electronic structure robust against the choice of exchange and correlation functional. Furthermore the calculated heat of formation describes the compound as metastable and suggests that it can be fabricated by tuning the relative Mg and N abundance during growth. Intriguingly the equilibrium lattice constant is close to that of MgO so that MgN is likely to form as an inclusion during the fabrication of N-doped MgO. If MgN can be made, the MgO/MgN system may become a materials platform for magnetic tunnel junctions not incorporating any transition metals
Assessment of density functional theory for iron(II) molecules across the spin-crossover transition
No full textOctahedral Fe2+ molecules are particularly interesting as they often exhibit a spin-crossover transition. In spite of the many efforts aimed at assessing the performances of density functional theory for such systems, an exchange-correlation functional able to account accurately for the energetic of the various possible spin-states has not been identified yet. Here, we critically discuss the issues related to the theoretical description of this class of molecules from first principles. In particular, we present a comparison between different density functionals for four ions, namely, [Fe(H2O)(6)](2+), [Fe(NH3)(6)](2+), [Fe(NCH)(6)](2+), and [Fe(CO)(6)](2+). These are characterized by different ligand-field splittings and ground state spin multiplicities. Since no experimental data are available for the gas phase, the density functional theory results are benchmarked against those obtained with diffusion Monte Carlo, one of the most accurate methods available to compute ground state total energies of quantum systems. On the one hand, we show that most of the functionals considered provide a good description of the geometry and of the shape of the potential energy surfaces. On the other hand, the same functionals fail badly in predicting the energy differences between the various spin states. In the case of [Fe(H2O)(6)](2+), [Fe(NH3)(6)](2+), [Fe(NCH)(6)](2+), this failure is related to the drastic underestimation of the exchange energy. Therefore, quite accurate results can be achieved with hybrid functionals including about 50% of Hartree-Fock exchange. In contrast, in the case of [Fe(CO)(6)](2+), the failure is likely to be caused by the multiconfigurational character of the ground state wave-function and no suitable exchange and correlation functional has been identified
Electron doping and magnetic moment formation in Nand C-doped MgO
No full textThe formation of the magnetic moment in C- and N-doped MgO is the result of a delicate interplay between Hund?s coupling, hybridization, and Jahn?Teller distortion. The balance depends on a number of environmental variables including electron doping. We investigate such a dependence by self-interaction corrected density functional theory and we find that the moment formation is robust with respect to electron doping. In contrast, the local symmetry around the dopant is more fragile and different geometries can be stabilized. Crucially the magnetic moment is always extremely localized, making any carrier mediated picture of magnetism in d0magnets unlikely
Electric Field Control of Valence Tautomeric Interconversion in Cobalt Dioxolene
No full textWe demonstrate that the critical temperature for valence tautomeric interconversion in cobalt dioxolene complexes can be significantly changed when a static electric field is applied to the molecule. This is achieved by effectively manipulating the redox potential of the metallic acceptor forming the molecule. Importantly, our accurate density functional theory calculations demonstrate that already a field of 0.1??V/nm, achievable in Stark spectroscopy experiments, can produce a change in the critical temperature for the interconversion of 20 K. Our results indicate a new way for switching on and off the magnetism in a magnetic molecule. This offers the unique chance of controlling magnetism at the atomic scale by electrical means
First Italian Consensus Conference on VATS lobectomy for NSCLC
No full textPurpose: Video-assisted thoracoscopic surgery ( VATS) lobectomy has become an accepted procedure for the treatment of selected cases of lung cancer. The aim of this project was to establish national practical recommendations for the management of patients suitable for VATS lobectomy.Methods: The Scientific Committee of the VATS Lobectomy Group ( a branch of the Italian Society of Thoracic Surgery) identified the consensus conference as an appropriate tool for a national debate. The consensus conference was organized following indications of the Italian Department of Health: a panel of experts reviewed the literature, the jury board revised the experts' reports, and the national conference discussed and voted on statements. The strength of recommendation for a statement was classified as weak, fair, or high when the total score ranged between 51% and 67%, 68% and 84%, or 85% and 100%, respectively.Results: Eighty-six Italian thoracic surgeons attended the 1st Italian Consensus Conference on VATS lobectomy in Giulianova, Italy, on October 29-30, 2015. Thirty-three topics were discussed: indications, surgical strategy, peri-operative management, and training were the main topics. Consensus was reached on 24 statements that were consequently recommended.Conclusions: The Italian Consensus Conference is the first attempt to discuss VATS lobectomy-related issues in a national scientific community. Such experience determined an improvement in epistemic knowledge among the Italian thoracic surgeons and could be a suggestion for other national communities
VATS GROUP. A new Italian experience|Il "VATS GROUP". Presentazione di una nuova esperienza italiana
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