Staff directory Asier Garcia Osa

Asier Garcia Osa

Research Support Technician
Theory and Simulation



  • Validity of the on-site spin-orbit coupling approximation

    Cuadrado R., Robles R., García A., Pruneda M., Ordejón P., Ferrer J., Cerdá J.I. Physical Review B; 104 (19, 195104) 2021. 10.1103/PhysRevB.104.195104. IF: 4.036

    Spin-orbit coupling (SOC) is generally understood as a highly localized interaction within each atom, whereby core electrons holding large J splittings transfer the SOC to the valence electrons of the same atom, while their direct impact on neighbor valence orbitals is usually small. Seivane and Ferrer [Phys. Rev. Lett. 99, 183401 (2007)PRLTAO0031-900710.1103/PhysRevLett.99.183401] proposed an approach within a tight-binding type ab initio framework assuming that the transfer of SOC from core to valence orbitals only takes place when both are on the same atom, leading to the so-called on-site approximation, which then has been successfully applied to a variety of systems. In this work we thoroughly test its general validity by confronting SOC related properties such as spin splittings, spin textures, or magnetic anisotropies calculated under the on-site approximation versus the more general approach where all the contributions to the SOC, including three-center integrals, are explicitly included. After considering a variety of systems with different dimensionalities, all presenting a strong SOC, we conclude that although the on-site approximation often provides accurate results, it breaks down in some systems where 5d electrons are close to the Fermi level due to their strong SOC and moderately large spatial extension. Furthermore, there are a few examples where subtle inaccuracies lead to qualitatively wrong conclusions, the most clear case being the doping of the topological surface state in Bi2Se3(0001). Finally, magnetic anisotropy energies calculated under this approximation tend to be underestimated. © 2021 American Physical Society.


  • Siesta: Recent developments and applications

    García A., Papior N., Akhtar A., Artacho E., Blum V., Bosoni E., Brandimarte P., Brandbyge M., Cerdá J.I., Corsetti F., Cuadrado R., Dikan V., Ferrer J., Gale J., García-Fernández P., García-Suárez V.M., García S., Huhs G., Illera S., Korytár R., Koval P., Lebedeva I., Lin L., López-Tarifa P., Mayo S.G., Mohr S., Ordejón P., Postnikov A., Pouillon Y., Pruneda M., Robles R., Sánchez-Portal D., Soler J.M., Ullah R., Yu V.W.-Z., Junquera J. The Journal of chemical physics; 152 (20): 204108. 2020. 10.1063/5.0005077. IF: 2.991

    A review of the present status, recent enhancements, and applicability of the Siesta program is presented. Since its debut in the mid-1990s, Siesta's flexibility, efficiency, and free distribution have given advanced materials simulation capabilities to many groups worldwide. The core methodological scheme of Siesta combines finite-support pseudo-atomic orbitals as basis sets, norm-conserving pseudopotentials, and a real-space grid for the representation of charge density and potentials and the computation of their associated matrix elements. Here, we describe the more recent implementations on top of that core scheme, which include full spin-orbit interaction, non-repeated and multiple-contact ballistic electron transport, density functional theory (DFT)+U and hybrid functionals, time-dependent DFT, novel reduced-scaling solvers, density-functional perturbation theory, efficient van der Waals non-local density functionals, and enhanced molecular-dynamics options. In addition, a substantial effort has been made in enhancing interoperability and interfacing with other codes and utilities, such as wannier90 and the second-principles modeling it can be used for, an AiiDA plugin for workflow automatization, interface to Lua for steering Siesta runs, and various post-processing utilities. Siesta has also been engaged in the Electronic Structure Library effort from its inception, which has allowed the sharing of various low-level libraries, as well as data standards and support for them, particularly the PSeudopotential Markup Language definition and library for transferable pseudopotentials, and the interface to the ELectronic Structure Infrastructure library of solvers. Code sharing is made easier by the new open-source licensing model of the program. This review also presents examples of application of the capabilities of the code, as well as a view of on-going and future developments.


  • Quasiparticle spectra of 2H-NbSe2: Two-band superconductivity and the role of tunneling selectivity

    Noat Y., Silva-Guillén J.A., Cren T., Cherkez V., Brun C., Pons S., Debontridder F., Roditchev D., Sacks W., Cario L., Ordejón P., García A., Canadell E. Physical Review B - Condensed Matter and Materials Physics; 92 (13, 134510) 2015. 10.1103/PhysRevB.92.134510. IF: 3.736

    We have studied the superconducting state of 2H-NbSe2 by scanning tunneling spectroscopy along two different crystal orientations, the c and the a/b axes. Along the c axis a large gap is dominant in the spectra, while a smaller gap is measured along the a/b axis. We show that these spectra are accurately described by the McMillan model where the small gap is induced through the coupling to the band associated with the large gap. In order to assign the small and large gaps to specific parts of the 2H-NbSe2 Fermi surface, the electronic structure was studied using first-principles calculations. While we cannot exclude the possibility of intrinsic anisotropy of the gaps, we propose that the large gap opens in the Fermi surface cylinders located around the corner K points while the sheets located around Γ are associated with the small gap. An additional component of the Fermi surface, a selenium based pocket, plays an essential role in the tunneling process. The role of the charge density wave occurring in this material is also discussed. Finally, we are able to give a coherent description of the observed characteristics of the tunneling spectra of 2H-NbSe2 as well as the differences with 2H-NbS2 where no charge density wave state is present. Further experimental work, such as high-resolution ARPES, would be very useful to confirm our interpretation. The approach and modeling developed here could also be relevant for other compounds of the dichalcogenide family. © 2015 American Physical Society.


  • Calculation of core level shifts within DFT using pseudopotentials and localized basis sets

    García-Gil S., García A., Ordejón P. European Physical Journal B; 85 (7, 239) 2012. 10.1140/epjb/e2012-30334-5.

    The calculation of core level shifts can be done in the context of density functional theory (DFT) using different approaches and physical approximations to the photoemission process. The initial state and the SCF approximations are the most commonly used ones. Here, we describe the details of their implementation in the context of DFT using pseudopotentials and localized atomic orbitals as a basis set, and in particular as applied to the Siesta code. We give a full account of the technicalities involved in these calculations, including the details of the ionic pseudopotential generation, basis sets, charge states and reference potential. We test the method by computing the core level shifts of the Si 2p level for a series of molecules and the p(2×2) asymmetric-dimer reconstruction of the Si(001) surface. © EDP Sciences, Societá Italiana di Fisica, Springer-Verlag 2012.


  • An efficient implementation of a QM-MM method in SIESTA

    Sanz-Navarro C.F., Grima R., García A., Bea E.A., Soba A., Cela J.M., Ordejón P. Theoretical Chemistry Accounts; 128 (4): 825 - 833. 2011. 10.1007/s00214-010-0816-5.

    We present the major features of a new implementation of a QM-MM method that uses the DFT code Siesta to treat the quantum mechanical subsystem and the AMBER force field to deal with the classical part. The computation of the electrostatic interaction has been completely revamped to treat periodic boundary conditions exactly, using a real-space grid that encompasses the whole system. Additionally, we present a new parallelization of the Siesta grid operations that provides near-perfect load balancing for all the relevant operations and achieves a much better scalability, which is important for efficient massive QM-MM calculations in which the grid can potentially be very large. © 2010 Springer-Verlag.


  • Optimal strictly localized basis sets for noble metal surfaces

    García-Gil S., García A., Lorente N., Ordejón P. Physical Review B - Condensed Matter and Materials Physics; 79 (7, 075441) 2009. 10.1103/PhysRevB.79.075441.

    The properties of the (111) surfaces of Cu, Ag, and Au are evaluated using a first-principles approach with strictly localized basis sets. These surfaces present metallic and extended properties that are a priori difficult to describe with a local-basis approach. We explore methodologies to enhance the basis sets of the surface atoms in order to accurately describe surface properties such as surface energies, surface states, and work functions. In this way, the advantages of local-basis computations (namely, efficiency, optimum size scaling, and a natural description of bonding in real space) can be retained, while keeping the accuracy in the description of the properties of the surface at a very good level. © 2009 The American Physical Society.


  • The SIESTA method; Developments and applicability

    Artacho E., Anglada E., Diéguez O., Gale J.D., García A., Junquera J., Martin R.M., Ordejón P., Pruneda J.M., Sánchez-Portal D., Soler J.M. Journal of Physics Condensed Matter; 20 (6, 064208) 2008. 10.1088/0953-8984/20/6/064208.

    Recent developments in and around the SIESTA method of first-principles simulation of condensed matter are described and reviewed, with emphasis on (i) the applicability of the method for large and varied systems, (ii) efficient basis sets for the standards of accuracy of density-functional methods, (iii) new implementations, and (iv) extensions beyond ground-state calculations. © 2008 IOP Publishing Ltd.