| 1. |
- Bhandary, Sumanta, et al.
(författare)
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Correlated electron behavior of metal-organic molecules : Insights from density functional theory combined with many-body effects using exact diagonalization
- 2016
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Ingår i: PHYSICAL REVIEW B. - 2469-9950. ; 93:15
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Tidskriftsartikel (refereegranskat)abstract
- A proper theoretical description of the electronic structure of the 3d orbitals in the metal centers of functional metalorganics is a challenging problem. We apply density functional theory and an exact diagonalization method in a many-body approach to study the ground-state electronic configuration of an iron porphyrin (FeP) molecule. Our study reveals that the consideration of multiple Slater determinants is important, and FeP is a potential candidate for realizing a spin crossover due to a subtle balance of crystal-field effects, on-site Coulomb repulsion, and hybridization between the Fe-d orbitals and ligand N-p states. The mechanism of switching between two close-lying electronic configurations of Fe-d orbitals is shown. We discuss the generality of the suggested approach and the possibility to properly describe the electronic structure and related low-energy physics of the whole class of correlated metal-centered organometallic molecules.
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| 2. |
- Bhandary, Sumanta, 1985-, et al.
(författare)
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Correlated electron behavior of metalorganic molecules: insights from density functional theory and exact diagonalization studies.
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The proper description of electronic structure of correlated orbitals in the metal centers of functional metalorganics is a challenging problem. In this letter, we apply density functional theory and exact diagonalization method in a many body approach to study the ground state electronic conguration of iron porphyrin (FeP) molecule. Our study reveals that FeP is a potential candidate for realizing a spin crossover due to a subtle balance of crystal elds and hybridization of the Fe d-orbitals and ligand N p-states. Moreover, the mechanism of switching between two close lying electronic congurations of Fe-d orbitals is revealed. This hybrid method can generally be applied to properly describe the electronic and related low energy physics of the whole class of correlated metal centered organometallic molecules.
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| 3. |
- Chang, Yueqing, et al.
(författare)
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Downfolding from ab initio to interacting model Hamiltonians : comprehensive analysis and benchmarking of the DFT+cRPA approach
- 2024
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Ingår i: npj Computational Materials. - 2057-3960. ; 10:1
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Tidskriftsartikel (refereegranskat)abstract
- Model Hamiltonians are regularly derived from first principles to describe correlated matter. However, the standard methods for this contain a number of largely unexplored approximations. For a strongly correlated impurity model system, here we carefully compare a standard downfolding technique with the best possible ground-truth estimates for charge-neutral excited-state energies and wave functions using state-of-the-art first-principles many-body wave function approaches. To this end, we use the vanadocene molecule and analyze all downfolding aspects, including the Hamiltonian form, target basis, double-counting correction, and Coulomb interaction screening models. We find that the choice of target-space basis functions emerges as a key factor for the quality of the downfolded results, while orbital-dependent double-counting corrections diminish the quality. Background screening of the Coulomb interaction matrix elements primarily affects crystal-field excitations. Our benchmark uncovers the relative importance of each downfolding step and offers insights into the potential accuracy of minimal downfolded model Hamiltonians.
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| 4. |
- Locht, Inka Laura Marie, 1986-
(författare)
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Cohesive and Spectroscopic properties of the Lanthanides within the Hubbard I Approximation
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- We describe the rare-earth elements using the Hubbard I approximation. We show that the theory reproduces the cohesive properties, like the volume and bulk modulus, and we find an excellent agreement between theory and experiment for the (inverse) photo emission spectra of the valence band. In addition we reproduce the spin and orbital moments of these elements. This licentiate thesis contains an introduction to the cohesive, magnetic and spectral properties of the rare-earth elements, to density functional theory and to density functional theory in combination with dynamical mean-field theory within the Hubbard I approximation. We also focus on some technical details, e.g. the optimal basis used in the electronic structure code and the role of charge self-consistency in properly describing the valence electrons.
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| 5. |
- Schobert, Arne, et al.
(författare)
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Ab initio electron-lattice downfolding : Potential energy landscapes, anharmonicity, and molecular dynamics in charge density wave materials
- 2024
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Ingår i: SciPost Physics. - 2542-4653. ; 16:2
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Tidskriftsartikel (refereegranskat)abstract
- The interplay of electronic and nuclear degrees of freedom presents an outstanding problem in condensed matter physics and chemistry. Computational challenges arise especially for large systems, long time scales, in nonequilibrium, or in systems with strong correlations. In this work, we show how downfolding approaches facilitate complexity reduction on the electronic side and thereby boost the simulation of electronic properties and nuclear motion—in particular molecular dynamics (MD) simulations. Three different downfolding strategies based on constraining, unscreening, and combinations thereof are benchmarked against full density functional calculations for selected charge density wave (CDW) systems, namely 1H-TaS2, 1T-TiSe2, 1H-NbS2, and a one-dimensional carbon chain. We find that the downfolded models can reproduce potential energy surfaces on supercells accurately and facilitate computational speedup in MD simulations by about five orders of magnitude in comparison to purely ab initio calculations. For monolayer 1H-TaS2 we report classical and path integral replica exchange MD simulations, revealing the impact of thermal and quantum fluctuations on the CDW transition.
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| 6. |
- Schobert, Arne, et al.
(författare)
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Downfolding the Su-Schrieffer-Heeger model
- 2021
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Ingår i: SciPost Physics. - 2542-4653. ; 11:4
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Tidskriftsartikel (refereegranskat)abstract
- Charge-density waves are responsible for symmetry-breaking displacements of atoms and concomitant changes in the electronic structure. Linear response theories, in particular density-functional perturbation theory, provide a way to study the effect of displacements on both the total energy and the electronic structure based on a single ab initio calculation. In downfolding approaches, the electronic system is reduced to a smaller number of bands, allowing for the incorporation of additional correlation and environmental effects on these bands. However, the physical contents of this downfolded model and its potential limitations are not always obvious. Here, we study the potential-energy landscape and electronic structure of the Su-Schrieffer-Heeger (SSH) model, where all relevant quantities can be evaluated analytically. We compare the exact results at arbitrary displacement with diagrammatic perturbation theory both in the full model and in a downfolded effective single-band model, which gives an instructive insight into the properties of downfolding. An exact reconstruction of the potential-energy landscape is possible in a downfolded model, which requires a dynamical electron-biphonon interaction. The dispersion of the bands upon atomic displacement is also found correctly, where the downfolded model by construction only captures spectral weight in the target space. In the SSH model, the electron-phonon coupling mechanism involves exclusively hybridization between the low- and high-energy bands and this limits the computational efficiency gain of downfolded models.
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| 7. |
- Schött, Johan
(författare)
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Theoretical and Computational Studies of Strongly Correlated Electron Systems : Dynamical Mean Field Theory, X-ray Absorption Spectroscopy and Analytical Continuation
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis encompasses theoretical and computational studies of strongly correlated elec-tron systems. Understanding how electrons in solids interact with each other is of great im-portance for future technology and other applications. From a fundamental point of view, the Coulomb interaction in a solid leads to a very challenging many-body problem, encapsulating many physical phenomena, e.g. magnetism. Treating this interaction, with a focus on local contributions, is the subject of this thesis. Both models and materials have been investigated, to obtain insight on the mechanisms determining the macroscopic properties of matter. This thesis is divided in four parts, each corresponding to a different project or topic.In the first project a many body method called dynamical mean field theory (DMFT) is used to study the paramagnetic phase of the Hubbard model. A stochastic version of the exact di-agonalization technique is developed for solving the effective impurity model arising in DMFT and generating real frequency spectral functions. In the next project, by combining density functional theory (DFT) with a static solution of the DMFT equations (DFT+U), magnetic ex-change interactions in transition metal oxides (TMOs) are investigated. The spin dependence of the functional is shown to be important for mapping magnetic excitations form the quantum mechanical system to a classical model.The next topic in this thesis concerns the x-ray absorption spectroscopy of TMOs. Spectral functions, in good agreement with experimental data, are calculated by combining DFT with multiplet ligand field theory (MLFT). The effects of the presence of a core-hole are studied in detail for NiO, as well as double counting issues related to higher order terms of the multiple ex-pansion of the Coulomb interaction. A strained induced linearly polarized spectrum is obtained for CaTiO3. Lastly, charge disproportionation is seen in Mo doped LaFeO3.Finally, a critical step in DMFT, called analytical continuation, to obtain physical observ-ables of interest is investigated. Analytical continuation means a transformation of a function in the complex plane. Several methods for performing this transformation are explained, and in particular steps for improving the robustness and accuracy of the Padé approximant method are described.
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| 8. |
- van Loon, Erik G.C.P., et al.
(författare)
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Coulomb engineering of two-dimensional Mott materials
- 2023
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Ingår i: npj 2D Materials and Applications. - 2397-7132. ; 7:1
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Tidskriftsartikel (refereegranskat)abstract
- Two-dimensional materials can be strongly influenced by their surroundings. A dielectric environment screens and reduces the Coulomb interaction between electrons in the two-dimensional material. Since in Mott materials the Coulomb interaction is responsible for the insulating state, manipulating the dielectric screening provides direct control over Mottness. Our many-body calculations reveal the spectroscopic fingerprints of such Coulomb engineering: we demonstrate eV-scale changes to the position of the Hubbard bands and show a Coulomb engineered insulator-to-metal transition. Based on our proof-of-principle calculations, we discuss the (feasible) conditions under which our scenario of Coulomb engineering of Mott materials can be realized experimentally.
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