CCP9 Conference posters
1. Chris Adriaanse, Computation of the free energy change in some steps in the reduction of O2 in aqueous solution
We use DFT based ab initio molecular dynamics and a self-interaction correction to calculate one electron oxidation of aqueous systems.
2. Simon Binnie, Benchmarking DFT Surface Energies with Quantum Monte Carlo
Previous work has demonstrated that for many materials, surface energies σ calculated by Density Functional Theory (DFT) methods appear to vary significantly depending on the exchange correlation (xc) functional used; This could pose significant problems when using DFT for predicting structures of nanocrystals both in vacuum and on substrates. Here we present initial results from a systematic study of σ for several ionic solids (LiH, LiF, NaF) using the VASP code and its projector augmented wave implementation of DFT. Calculations were performed using the four functionals available in VASP (LDA, PW91, PBE and RPBE) along with the recent WuCohen modification of PBE. The results we present show that there is indeed a significant variation in σ due to differing functionals. Furthermore we are able to ascertain which functionals gave the most accurate results by performing calculations of σ for LiH using diffusion quantum Monte Carlo methods which are generally accepted to be significantly more accurate, albeit more expensive, in calculating these quantities.
3. Giuseppe Mallia, Doping Titania with transition metal to make a dilute ferromagnetic semiconductor: insights from hybrid density functional simulations
Recently, it has been observed that when doping TiO2 with transition metal (TM) impurities at low concentration (Ti 1-xMxO2, with X=TM and x from 0.01 upto 0.14), titanium dioxide exhibits room temperature (RT) ferromagnetism [1,2]. The origin of the RT ferromagnetism has been studied using a number of experimental techniques but there is still no clear consensus about the resultant lattice structure, the sites adopted by TM ions, the distribution of the ions in the lattice, their oxidation state or the magnetic moment per ion.
The magnetic ground state of Fe-doped and Co-doped TiO2 has been investigated within the hybrid-exchange approximation to density-functional theory [3]. A detailed study of the nature and stability of the predicted ground state with respect to variations in the oxidation state of the transition metal ion, the delocalisation or self trapping of holes donated to the lattice and the treatment of electronic exchange and correlation is presented.
[1] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, and H. Koinuma, science 291, 854 (2001a).
[2] Y. Matsumoto, R. Takahashi, M. Murakami, T. Koida, X.J. Fan, T. Hasegawa, T. Fukumura, M. Kawasaki, S. Koshihara, and H. Koinuma, Jpn. J. Appl. Phys. 40, L1204 (2001b).
[3] G. Mallia and N.M. Harrison, Phys. Rev. B, 75, 165201 (2007)
4. Gareth Conduit, DMC calculations in valley degenerate semiconductors
A many-flavor electron gas (MFEG) in a semiconductor with a valley degeneracy ranging between 6 and 24 was analyzed using diffusion Monte Carlo (DMC) calculations. The DMC results compare well with an analytic expression derived by one of us [Phys. Rev. B 78, 035111 (2008)] for the total energy to within 1% over an order of magnitude range of density, which increases with valley degeneracy. For Bi2Te3 (six-fold valley degeneracy) the applicable charge carrier densities are between 7*10^19cm-3 and 2*10^20cm^-3. DMC calculations distinguished between an exact and a useful approximate expression for the 24-fold degenerate MFEG polarizability for wave numbers 2p_F<q<7p_F. The analytical result for the MFEG is generalized to inhomogeneous systems by means of a gradient correction, the validity range of this approach is obtained. Employed within a density-functional theory calculation this approximation compares well with DMC results for a quantum dot.
5. Nicholas Hine, Supercell Size Scaling of Formation Energies of Charged Defects
We present DFT calculations of formation energies of charged defects in Al2O3. We present a new method to eliminate the dependence of defect cell total energies on supercell size and shape.
6. Giulia C De Fusco, Ab Initio Study of the Electronic Structure and Magnetic Properties of the High-Temperature V(TCNE)2 Organic-Based Magnet
In the past two decades organic and molecular magnets have captured increasing atten- tion due to their unique properties which make them optimum candidates for many inno- vative technological applications [1]. Among the vast family of hybrid molecular magnets, V(TCNE)2 (TCNE = tetracyanoethylene) represents a major breakthrough being a com- pletely spin polarised semiconductor which exhibits ferrimagnetic behaviour above room temperature (TC ∼ 400 K) [2]. Many commercial applications have been envisioned for this material including its use for magnetic memories, magnetic shielding and spintronics devices [3]. Due to its insolubility, extreme air and water sensitivity and long-range struc- tural disorder, V(TCNE)x crystalline structure is yet to be characterised and its electronic and microscopic magnetic properties remain unknown. The present work proposes a realistic model structure of the organic-based magnet [V(TCNE)2 ] and provides an explanation for the exceptional magnetic behaviour showed by this material by analysing in detail the struc- tural and electronic properties computed for the model system. All the calculations have been performed using the hybrid exchange density functional B3LYP in periodic bound- ary conditions as implemented within the CRYSTAL06 package [4]. The application of this scheme has showed to be suitable for the investigation of the magnetic interactions of strongly correlated materials [5].
[1] J. S. Miller and A. J. Epstein, Angew. Chem. Int. Ed. Engl. 33, 385 (1994, and references therein).
[2] J. M. Manriquez, G. T. Yee, R. S. McLean, A. J. Epstein, and J. S. Miller, Science 252, 1415 (1991).
[3] B. G. Morin, C. Hahm, A. J. Epstein, and J. S. Miller, J. Appl. Phys. 75, 5782 (1994).
[4] R. Dovesi, V. Saunders, C. Roetti, R. Orlando, C. Zicovich-Wilson, F. Pascale, B. Civalleri, K. Doll,
N. Harrison, I. Bush, et al., CRYSTAL06, Universit` di Torino (Torino, 2006).
[5] W. C. Mackrodt, N. M. Harrison, V. R. Saunders, N. L. Allan, M. D. Towler, E. Apra, and R. Dovesi,
Philos. Mag. A 68, 653 (1993).
7. Leandro Liborio, Magneli Phases: An ab initio Approach
The system titanium-oxygen plays a fundamental role in the study of non-stoichiometry. Rutile only exists in a non-stoichiometric TiO2-x form [1]. For low x, the point defects that dominate are oxygen vacancies and titanium interstitials [2, 3]. But, as the sample is reduced, complex defects structures form: the socalled Magnéli phases [4, 5], which have a TinO2n-1 stoichiometry. Experimental observations have showed e that these complex defects are extended defects [4, 6], known as Crystallographic Shear planes.
In this work we studied the electronic structure of these phases and used ab initio thermodynamics [7] to calculate the formation energies, at different environmental conditions, of different oxygen-defective structures in rutile.
Magnéli phases with 3≤n≤5 and the two fundamental points defects -Ti interstitials and neutral oxygen vacancies- were studied.
To simulate our systems from ab initio, massive parallel calculations were necessary and these were performed using the CASTEP and CRYSTAL ab initio codes. Some of the calculations made use of the facilities of HPCx while others were performed on the computing resources provided by STFCs e-Science facility.
References
[1] P. Waldner and G. Eriksson, Calphad , v. 23, p. 189, 1997.
[2] I. M. Mazharul, T. Bredow and A. Gerson, Phys. Rev. B , v. 76, p. 045217, 2007.
[3] E. Cho, S. Han,[4] L. A. Bursill and B. G. Hyde, Prog. Solid State Chem., v. 7, p. 177, 1972.
[5] S. Andersson and L. Jahnberg, Arkiv för kemi, v. 21, p. 413, 1963.
[7] K. Reuter and M. Scheffler, Phys. Rev. B, v. 65, p. 035406, 2001.
8. Andrew Morris, Defects in Semiconductors using Random Structure Searching
I shall give a description of the method of random structure searching as applied to hydrogen/silicon complexes in bulk silicon. We consider a defect with a single self-interstitial silicon atom and up to four hydrogen atoms studied within density-functional theory.
We search for low-energy defects by starting from an ensemble of structures in which the atomic positions in the defect region have been randomised. We then relax each structure to a minimum in the energy. We find a new defect consisting of a self-interstitial and one hydrogen atom which has a higher symmetry and a lower energy than previously reported structures. We recover the 2-hydrogen/silicon defect found in previous studies and confirm that it is the most stable such defect. Our best 3-hydrogen/silicon defect has a slightly different structure and lower energy than the one previously reported, and our lowest energy 4-hydrogen/silicon defect is different to those of previous studies.
9. Arash Mostofi, Linear-Scaling DFT+U
DFT+U augments the description of on-site Coulomb interactions offered by standard exchange-correlation functionals hence extending their range of applicability. We present an implementation of DFT+U within the linear-scaling ONETEP code.
10. Jon Swaim, The Electronic Structure of CrO2 Revisited: A Hybrid-Exchange Density Functional Theory Study
Spintronics represent a new class of electronic devices, based on their ability to conduct spin-polarized currents. CrO2 has received much attention as a potential spintronic material due to its theoretical 100% spin-polarization. Band structure calculations based on the local density approximation (LDA) to density functional theory (DFT) predict CrO2 to be a half-metallic ferromagnet, in which the density of states (DOS) at the Fermi energy (EF) is completely spin-polarized with a large density of spin-up electons and a wide band gap in the spin-down spectrum. However, there still exists controversy over the underlying physical picture of CrO2 , and it remains puzzling that a very low photoemission intensity is observed at EF.
We present electronic structure calculations of bulk CrO2 using hybrid-exchange density functional theory methods, B3LYP and PBE0. We demonstrate that such approaches are able to predict the electronic and magnetic properties of strongly correlated CrO2 . Inclu- sion of the Fock exchange amounted to a better understanding of electron correlation via reduction of the self-interaction of strongly correlated 3d electrons inherent in LDA-based functionals. In our work, this has amounted to improved predictions of the minority-spin band gap and d-d electron correlation effects. In particular, we observe Hubbard splitting of the Cr-3d bands in excellent agreement with recent photoemission work by Ventrice et al [1]. We emphasize that future theoretical methods should include an accurate treatment of electron-correlation effects in order to properly understand the nature of CrO2 . The electronic structure of the (110) surface of CrO2 is also discussed.
[1] C. A. Ventrice, J. Phys.: Condens. Matter 19 (2007) 315207
11. Dzidka Szotek, Application of Local Self-Interaction Correction (SIC) to Transition Metal Oxides
We apply the self-interaction corrected local spin density (SIC-LSD) approximation, implemented locally within the Kohn-Korringa-Rostoker (KKR) multiple scattering method, to study electronic and magnetic properties of transition metal (TM) oxides both at T=0 K and finite temperatures. While the local self-interaction correction (LSIC) improves on the LSD description of the localized TM 3d-electrons, the thermally induced magnetic fluctuations are treated using a mean field 'disordered local moment' (DLM) approach and at no stage is there a fitting to an effective Heisenberg model. With this combined approach of LSIC and DLM we reproduce the experimental ordering tendencies, as well as the qualitative trend in ordering temperatures. In addition, we obtain a large insulating band gap in the paramagnetic state which hardly changes with the onset of magnetic order as the temperature is lowered from the above of the Neel temperature. We also find good agreement with experiment for the ground state AFII magnetic moments, lattice parameters and band gaps across the TM oxides series.
12. Claudia Utfeld, CoxFex?1S2 : How close to half-metallicity?
CoS2 is a material thought to be very close to being a half-metal, i.e. to only be conducting in one spin. We present density functional calculations of the electronic structure of CoxFex?1S2 and make comparison with experimental results from x-ray magnetic Compton scattering.
13. Rebecca Varns, A First Principles Study of Group 13 Simple Metal Clusters: Superatom or Cluster?
Simple metal clusters are of particular interest due to their unique electronic properties. However, there are several forms of simple metal cluster that possess superatomic properties. The atoms within a superatom have an excess of electrons, these are delocalised around the cluster and occupy a new set of orbitals. These orbitals are defined by the group of atoms in the cluster rather then each individual atom separately.
Aluminium clusters (Al13 for example) have been shown to form superatoms, whereas its boron counterpart does not. Within this project we intend to address the following question; What are the microscopic electronic properties of a cluster that determine whether or not it will behave as a superatom? Here we will present preliminary results on this project. Plane wave pseudopotential calculations [3] of charged and neutral, Group thirteen, six atom clusters in a square bi-pyramidal structure will be presented. Changes in electronic structure and bonding will be shown and discussed in terms of charge density differences and spin density slices.
14. Rebecca Varns, A First Principles Study of the Electronic Structure and Chemical Bonding of Neutral and Charged Boron Clusters.
In recent years the study of the chemical and physical properties of metal clusters has been of great interest. Boron possesses a diverse and complex range of chemistry and the polyhedral patterns that characterise boron cluster chemistry has presented a challenge to understanding for many years. Here we present plane wave pseudopotential calculations of charged and neutral six atom boron clusters. Several stable structures were found with the global minimum for B6 and B6- forming a planar structure and B6+ forming a three dimensional cluster. These structures will be discussed in terms of the results of density functional calculations and illustrated with electron and spin density maps of the ground state structures of the clusters.
15. Rudolf Zeller, A Reformulation of the KKR Green-Function Method for Large-Scale Density-Functional Calculations
It is shown how sparsity of the tight-binding Korringa-Kohn-Rostoker (KKR) matrices together with iterative solution techniques and finite-temperature complex energy integration can be used to reduce the computational effort which normally increases cubically with system size.
16. Nicholas Zonias, Large-Scale DFT Calculations on Silicon Nanocrystals
We present preliminary calculations of electronic and structural properties of hydrogen passivated silicon nanocrystals.
17. Walter Temmerman, Finite Temperature Magnetism of Heavy Rare Earths: SIC-DLM Study
We combine the Local Self-Interaction Correction (LSIC), implemented in the Kohn-Korringa-Rostoker (KKR) multiple scattering theory (MST), with a mean-field 'disordered local moment' (DLM) approach, to study the finite temperature magnetism of heavy rare earths. Treating Gd as a prototype we calculate a paramagnetic susceptibility as a function of volume and c/a ratio and observe changes in the magnetic structure, mapping out a universal phase diagram of the heavy rare earths magnetism. We show that the phase diagram has predictive power, as knowing lattice parameters one can determine the magnetic structure of various heavy rare earth alloys. Also, the calculated ordering temperatures and wave vectors are in good agreement with experiment. We verify the importance of the c/a ratio, but discover that the lanthanide contraction plays a separate, completely distinct role in determining the magnetic properties of the heavy rare earth elements. The incommensurate magnetic ordering is driven by the Fermi surface nesting, as the conduction electrons mediate the interactions between the spin magnetic moments set up by the localized f-electrons.
18. Calvin Davidson, A Density Functional Theory Study of the Adsorption of Tetracene on Reconstructed Oxygen Terminated Copper (110) Surfaces
Organic semiconductors are already employed in electronic devices, such as field-effect transistors and organic light-emitting diodes. Their potential for applications in micro and opto-electronics as thin and even flexible films holds great promise for the future. Polyacenes, in particular, anthracene, pentacene, and tetracene are among the most extensively studied of the organic semiconductors. The early stages of this work examines the potential energy surface describing the adsorption of tetracene on a reconstructed oxygen terminated copper (110) substrate and shows that our calculations for possible adsorption sites identify a preferred site, agreeing with experimental evidence from scanning tunnelling microscopy and also revealing that the adsorption appears to be nonparallel in nature.
19. Duncan Riley, Hydrogen Storage in Metal-Organic Frameworks
We present studies of hydrogen absorption in MOFs. Results include binding energies and rotational spectra from H2 along with phonon calculations with bound atomic H motivated by the proposed spillover mechanism.
20. Mark Robinson, Ab initio Study of Peptide Stacking in an Amyloid Fibril
A number of possible TTR fibril structures have been constructed optimised and ranked using the CASTEP implementation of DFT. Chemical shifts have also been computed to compare the structures and determine the atomic structure of the fibril.
21.Fabien Tran, PBE+U Calculations of the Jahn-Teller Effect in PrO2
The Jahn-Teller distortion in praseodymium dioxide PrO2 has been studied using density functional theory with the PBE+U functional for the exchange-correlation energy as implemented into the WIEN2k code [1].
Experimentally, several possible structures were suggested based on neutron diffraction experiments [2]. Among these possible structures, we could identify the structure with the lowest energy [3]. The magnitude of distortion (displacement of the oxygen sublattice) for this structure is in very good agreement with the experimental value. Good agreement with experiment is also obtained for the electronic and magnetic properties of PrO2. More recently, the same distorted structure was unambiguously determined by experiment [4] and theoretical calculations using a model Hamiltonian [5].
[1] P. Blaha et al., WIEN2k, An Augmented Plane Wave Plus Local Orbitals Program for Calculating Crystal Properties (Vienna University of Technology, Austria, 2001).
[2] C. H. Gardiner et al., Phys. Rev. B 70, 024415 (2004).
[3] F. Tran et al., Phys. Rev. B 77, 085123 (2008).
[4] C. H. Webster et al., Phys. Rev. B 76, 134419 (2007).
[5] J. Jensen, Phys. Rev. B 76, 144428 (2007).
