National Repository of Grey Literature 13 records found  1 - 10next  jump to record: Search took 0.01 seconds. 
Tensor Network-based Computational Methods for Strongly Correlated Molecular Quantum Mechanics
Brandejs, Jan ; Veis, Libor (advisor) ; Mančal, Tomáš (referee) ; Pitoňák, Michal (referee)
Despite an immense progress in recent decades, a precise treatment of strongly corre- lated molecular systems still remains a challenge as of today. To help solve this problem, we have developed a massively parallel implementation of DMRG, called MOLMPS. For nonrelativistic systems requiring accurate treatment of both static and dynamical corre- lation, we have extended MOLMPS by the means of the almost linear scaling DLPNO- TCCSD method. In relativistic domain, we are the first who employed the 4c-CCSD to add dynamical correlation on top of DMRG, yielding the 4c-TCCSD method. When tested on benchmarks like the π-conjugated anthracene tetramer with CAS(63,63) and the FeMoco cofactor with CAS(113,76). We showed a good parallel performance on up to about 2000 CPUs. On the example of Iron(II)-Porphyrin model, we showed that the DLPNO-TCCSD captures 99.9% of TCCSD correlation energy. Our spectroscopic study on heavy diatomics showed that the 4c-TCCSD approach increases the precision of underlying CCSD to the order of CCSD(T) and that it is a promising approach. The thesis discusses three different implementations of quantum chemical methods based on QC-DMRG. 1
Ab initio calculations of BN molecule and small fused hydrocarbons with single-reference and Brillouin-Wigner multireference coupled clusters methods
Veis, Libor
The topic of the diploma thesis is calculations with single-reference and Brillouin-Wigner multireference coupled clusters methods. In the beginning, the brief overview of the theory of selected quantum chemical methods with emphasis on the theory of coupled clusters (single reference as well as multireference) is presented. Third chapter deals with calculations of the lowest lying singlet and triplet states of boron nitride molecule (BN). The lowest singlet state (1+) exhibits multireference character and this molecule is thus a suitable candidate for testing of newly developed computational methods. Vibrational frequencies, anharmonicities and singlet-triplet energy splitting were calculated with different variants of multireference Brillouin-Wigner coupled clusters method (MR BWCC). The results were compared with experimental values and the results of single-reference and reduced multireference coupled clusters. The fourth and fifth chapters present the study of a series of seven small fused cyclopentanes of the formula C5H2n (n = 0 - 4) [1]. The existence of local minima of all seven structures was verified at the CCSD(T)/cc-pVTZ level. Their stability was predicted according to enthalpies of formation and strain energies. 1H and 13C NMR spectra (chemical shifts), IR and Raman spectra were...
Short-lived Delocalization and Absorption by Light
Vokrouhlický, David ; Mančal, Tomáš (advisor) ; Veis, Libor (referee)
Coherent exciton delocalization improves the light harvesting function of photosyn- thetic antennae by creating conditions for very fast excitation transfer in space. This thesis focuses on two different effects creating coherence - short-lived excitation by light and weak coupling between pigments that is present in the system on longer timescales. The evolution and relaxation of simple systems - the dimer and trimer - are calculated. The core of this thesis are newly developed numerical methods for distinguishing and quantifying the effect of the two types of coherence throughout evolution, which are applied to the aforementioned systems. 1
Electronic structure calculations of biologically relevant transition metal complexes
Matoušek, Mikuláš ; Veis, Libor (advisor) ; Mančal, Tomáš (referee)
Porphyrins are an important class of biomolecules, which are heavily studied, both ex- perimentally and computationally. But, despite the intensive efforts, for many questions we still aren't able to consistently find an agreement between theory and experiment. One of the still unresolved issues is the character of the ground state of the Fe(II)-porphyrin molecule. We used a model of the Fe(II)-porphyrin molecule to study the effects of geometrical changes on the spin states. By carrying out extensive DMRG-CASSCF cal- culations topped with TCCSD correlation treatment we are able to link the effects of these geometrical changes to the experimental results, and predict a quintet ground state for the isolated Fe(II)-porphyrin molecule. Also, using a ligated porphyrin belonging to the iron porphyrin carbene class of molecules, we demonstrate by combining the CASSCF and AC0 methods that geometrical changes outside the porphyrin core cannot be over- looked. 1
Structure and selforganization of aggregates of photosynthetic molecules
Sláma, Vladislav ; Mančal, Tomáš (advisor) ; Duffy, Christopher (referee) ; Veis, Libor (referee)
In this work we demonstrate how quantum chemistry (QC) methods and molecular dynamics (MD) simulations can be used in combination with Frenkel exciton model (FEM) to obtain optical and excitation energy transfer properties of complex molecular systems from the molecular structure. The combination of QC and MD methods with FEM provides a powerful tool to study and explain molecular level processes, which are out of reach of the standard FEM parametrization. We use these methods to study and explain molecular mechanism of excitation energy transfer in rylene dyads, especially to explain observed fast excitation energy transfer in dyad with orthogonal arrangement of transition dipoles, where standard approach predicts no excitation energy transfer. On a fundamental level, we relate FEM to configuration interaction method of QC and propose extension of FEM, which accounts for interaction between excitonic manifolds. We investigate effects of this interaction on the optical properties. Inspired by the core features of FEM, we propose new concept of artificial light harvesting antenna based on fluorographene, with design principles inspired by natural light harvesting complexes. We use structure based methods to investigate its excitation transfer properties. We also introduce a new general method for treating...
Modelling of Ultracold Gases in Multidimensional Optical Lattices
Urbanek, Miroslav ; Soldán, Pavel (advisor) ; Opatrný, Tomáš (referee) ; Veis, Libor (referee)
Title: Modelling of Ultracold Gases in Multidimensional Optical Lattices Author: Miroslav Urbanek Department: Department of Chemical Physics and Optics Supervisor: doc. Ing. Pavel Soldán, Dr. Abstract: Optical lattices are experimental devices that use laser light to confine ultracold neutral atoms to periodic spatial structures. A system of bosonic atoms in an optical lattice can be described by the Bose-Hubbard model. Although there exist powerful analytic and numerical methods to study this model in one dimension, their extensions to multiple dimensions have not been as successful yet. I present an original numerical method based on tree tensor networks to simulate time evolution in multidimensional lattice systems with a focus on the two-dimensional Bose-Hubbard model. The method is used to investigate phenomena accessible in current experiments. In particular, I have studied phase collapse and revivals, boson expansion, and many-body localization in two-dimensional optical lattices. The outcome of this work is TEBDOL - a program for modelling one-dimensional and two-dimensional lattice systems. Keywords: Bose-Hubbard model, multidimensional system, optical lattice, tensor network

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