Density matrix embedding: A strong-coupling quantum embedding theory

We extend our density matrix embedding theory (DMET) [Phys. Rev. Lett. 109 186404 (2012)] from lattice models to the full chemical Hamiltonian. DMET allows the many-body embedding of arbitrary fragments of a quantum system, even when such fragments are open systems and strongly coupled to their environment (e.g., by covalent bonds). In DMET, empirical approaches to strong coupling, such as link atoms or boundary regions, are replaced by a small, rigorous quantum bath designed to reproduce the entanglement between a fragment and its environment. We describe the theory and demonstrate its feasibility in strongly correlated hydrogen ring and grid models; these are not only beyond the scope of traditional embeddings, but even challenge conventional quantum chemistry methods themselves. We find that DMET correctly describes the notoriously difficult symmetric dissociation of a 4x3 hydrogen atom grid, even when the treated fragments are as small as single hydrogen atoms. We expect that DMET will open up new ways of treating of complex strongly coupled, strongly correlated systems in terms of their individual fragments.Comment: 5 pages, 4 figure

Gemeinsame Ressourcennutzung mit Hilfe einer unternehmensübergreifenden Kapazitätsbörse - Eine empirische Analyse

Der steigende Wettbewerbsdruck bedingt durch globale Konkurrenz und sich wandelnde Rahmenbedingungen zwingt besonders mittelständische Unternehmen im produzierenden Gewerbe ihre Ressourcen effizient einzusetzen. Eine vielversprechende Möglichkeit hierfür ist die Teilnahme an einer unternehmensübergreifenden Kapazitätsbörse, die auf Betriebsmittel abzielen und eine weitere Rationalisierung von Produktionsfaktoren in Form der Harmonisierung ihrer Auslastung bewirken. Eine Kapazitätsbörse soll die Transparenz und Flexibilität der Auftragsvergabe bzw. -annahme einzelner Engpass- bzw. Leerlaufprozesse ermöglichen. In einer Umfrage unter regionalen Unternehmen sind Rahmenbedingungen und Anforderungen hinsichtlich einer unternehmensübergreifenden Kapazitätsbörse erarbeitet worden

The intermediate and spin-liquid phase of the half-filled honeycomb Hubbard model

We obtain the phase-diagram of the half-filled honeycomb Hubbard model with density matrix embedding theory, to address recent controversy at intermediate couplings. We use clusters from 2-12 sites and lattices at the thermodynamic limit. We identify a paramagnetic insulating state, with possible hexagonal cluster order, competitive with the antiferromagnetic phase at intermediate coupling. However, its stability is strongly cluster and lattice size dependent, explaining controver- sies in earlier work. Our results support the paramagnetic insulator as being a metastable, rather than a true, intermediate phase, in the thermodynamic limit

Visualizing Complex-Valued Molecular Orbitals

We report an implementation of a program for visualizing complex-valued molecular orbitals. The orbital phase information is encoded on each of the vertices of triangle meshes using the standard color wheel. Using this program, we visualized the molecular orbitals for systems with spin-orbit couplings, external magnetic fields, and complex absorbing potentials. Our work has not only created visually attractive pictures, but also clearly demonstrated that the phases of the complex-valued molecular orbitals carry rich chemical and physical information of the system, which has often been unnoticed or overlooked

Cellular mechanisms of organ-specific metastasis of Ewing's sarcoma

Ewing's sarcoma is the second most common bone tumour in children and adolescents. The prognosis is mainly influenced by the occurrence of primary metastasis. Although great improvement in treatment has been achieved, still only 2/3 of patients with localized disease can be cured. Furthermore, the 3-year event free survival in patients with lung metastases is only ~50%, and is less than 20% in patients with bony metastases. Metastatic models of Ewing’s sarcoma developed in this study using cell lines in immunocompromised mice show a pattern of disease spread similar to that found in patients, providing a suitable system for studying the metastatic process likely occurring in the course of Ewing’s sarcoma. The comparison of microarray gene expression patterns revealed interesting candidate genes for diagnosis and identified putative metastasis-specific targets that might be exploited in the development of new treatment approaches. However, it will be necessary to additionally analyse these patterns in primary material. One gene that formerly has been shown to play a role in the metastasis to bones in a variety of cancer types is CXCR4, which encodes for the cytokine receptor of CXCL12 (SDF-1), and which plays a role in the metastasis to bones in a variety of other cancer types. As Ewing’s sarcoma cells express CXCR4, a shRNA vector was constructed, transduced and stably expressed to investigate the role of the CXCR4/CXCL12 axis in Ewing’s sarcoma cells via RNA interference. This stability provides the possibility of an in vitro and furthermore an in vivo use for investigations. In order to investigate the biology of bone malignancy and especially the interaction of tumour cells with cells of the microenvironment of the bone directly, an orthotopic model for Ewing’s sarcoma was developed. Additionally, osteosarcoma as a further primary bone sarcoma and prostate carcinoma as a cancer type with frequent bone metastases were tested in this model. The previously described technique of intrafemoral transplantation was used in this model. Using small animal imaging techniques such as nano computed tomography and magnetic resonance imaging in combination with histology it could be shown that the transplanted cells led to the development of orthotopic tumours presenting a comparable picture to the clinical situation. This model will be further used for research projects performed in the Northern Institute for Cancer Research on the effectiveness of drugs targeting Ewing’s sarcoma cells.EThOS - Electronic Theses Online ServiceDeutsche Krebshilfe : Bone Cancer Research Trust : North of England's Children's Cancer Research Fund : Newcastle Healthcare CharityGBUnited Kingdo

Density matrix embedding: A simple alternative to dynamical mean-field theory

We introduce DMET, a new quantum embedding theory for predicting ground-state properties of infinite systems. Like dynamical mean-field theory (DMFT), DMET maps the the bulk interacting system to a simpler impurity model and is exact in the non-interacting and atomic limits. Unlike DMFT, DMET is formulated in terms of the frequency-independent local density matrix, rather than the local Green's function. In addition, it features a finite, algebraically constructible bath of only one bath site per impurity site, which exactly embeds ground-states at a mean-field level with no bath discretization error. Frequency independence and the minimal bath make DMET a computationally simple and very efficient method. We test the theory in the 1D and 2D Hubbard models at and away from half-filling, and we find that compared to benchmark data, total energies, correlation functions, and paramagnetic metal-insulator transitions are well reproduced, at a tiny computational cost.Comment: 5 pages, 5 figure

Automated construction of molecular active spaces from atomic valence orbitals

We introduce the atomic valence active space (AVAS), a simple and well-defined automated technique for constructing active orbital spaces for use in multiconfiguration and multireference (MR) electronic structure calculations. Concretely, the technique constructs active molecular orbitals capable of describing all relevant electronic configurations emerging from a targeted set of atomic valence orbitals (e.g., the metal d orbitals in a coordination complex). This is achieved via a linear transformation of the occupied and unoccupied orbital spaces from an easily obtainable single-reference wave function (such as from a Hartree–Fock or Kohn–Sham calculations) based on projectors to targeted atomic valence orbitals. We discuss the premises, theory, and implementation of the idea, and several of its variations are tested. To investigate the performance and accuracy, we calculate the excitation energies for various transition-metal complexes in typical application scenarios. Additionally, we follow the homolytic bond breaking process of a Fenton reaction along its reaction coordinate. While the described AVAS technique is not a universal solution to the active space problem, its premises are fulfilled in many application scenarios of transition-metal chemistry and bond dissociation processes. In these cases the technique makes MR calculations easier to execute, easier to reproduce by any user, and simplifies the determination of the appropriate size of the active space required for accurate results