Mitas_RecentApplications - Quantum Monte Carlo methods...

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Quantum Monte Carlo methods: recent developments and applications Lucas Wagner, Michal Bajdich, Gabriel Drobny Zack Helms, Lubos Mitas North Carolina State University in collab. with Jeffrey Grossman, UC Berkeley Kevin E. Schmidt, Arizona State U. [email protected] Urbana, August 2006
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MnO solid calculations: paradigmatic TMO antiferromagnet [email protected] - large correlation effects, difficult to treat by traditional methods: competition of Coulomb, exchange, correlation and crystal-field effects - supercells with 8, 16, 20 and 24 atoms: up to ~ 250 valence electrons - gap estimations using promotion from valence -> to conduction band
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[email protected] Band structure of MnO by HF and DFT: UHF B3LYP PW91
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[email protected] MnO cohesion and band gap Cohesive energy [eV] HF B3LYP DMC Exp. 6.0 10.2 9.40(5) 9.5 Band gap: B -> Γαμ μ α excitation HF B3LYP DMC Exp. 14.2 4.0 4.8(3) 4.2 Small bias towards higher energy for the excited state
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[email protected] BaTiO3 cohesion Cohesive energy [eV] DFT/PW91 DMC/HF DMC/DFT Exp. 38.0 30.9(4) 31.2(4) 31.56 Note: cohesion comes very close to experiment from a straightforward application of QMC Current effort: ferroelectric distortion
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[email protected] Electronic structure of hexaborides CaB 6 , La x Ca 1-x B 6 , . .. 3% La-doped CaB 6 is a weak magnet up to 600-900K (!) No d or f electrons : - genuine itinerant magnetism ? Calculations inconclusive: semimetal or insulator Undoped CaB 6 : insulator ? exitonic insulator ? metal ? Experiments contardictory: both metallic and insulating behavior observed Can we calculate the gap before the experiment ?
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[email protected] CaB 6 band structure in Hartree-Fock Large gap of the order of 7 eV
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[email protected] CaB 6 band structure in DFT - B3LYP Gap is now only about 0.5 eV
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[email protected] CaB 6 band structure in DFT - PW91 1 eV overlap at the X point: d-states on Ca Fixed-node DMC gap: 1.2(2) eV X Γ
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Oxygen 2p-3d hybridized orbitals: HF vs B3LYP, ie, charge transfer vs covalent bonding [email protected] Hartree-Fock B3LYP O Mn Mn O
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FNDMC with B3LYP orbitals within 0.2 eV of experiment Exp MnO binding by various methods [email protected]
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Silicon nanocrystals: ultrabright luminescence quantum dots Collaboration with experimental group at U. Illinois (M.Nayfeh) and LLNL group (Grossman, Williamson, Puzder) - unlike bulk silicon, Si nanocrystals have interesting optical properties: strong luminescence in the visible range - can be prepared in large quantities and in reasonably homogeneous batches of sizes from 1 nm ( "blue" ) to about 2.7 nm ( "red" ), "magic sizes" - nanocrystals are stable with hydrogen passivated or reconstructed surfaces - lots of interest: sensors, optical devices, biomarkers,. .. Si
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