National Repository of Grey Literature 25 records found  1 - 10nextend  jump to record: Search took 0.01 seconds. 
Coupled clusters tailored by matrix product state wave functions
Antalík, Andrej ; Pittner, Jiří (advisor) ; Noga, Jozef (referee) ; Soldán, Pavel (referee)
The central problem in the modern electronic structure theory is the calculation of cor- relation energy, possibly by an approach that would account for both static and dynamic correlation in an efficient, balanced and accurate way. In this thesis, I present a collection of methods that combine the effective treatment of dynamic correlation by the coupled cluster theory with density matrix renormalization group, a well-established technique for calculations of strongly correlated systems. The connection between them is achieved via the tailored coupled clusters (TCC) ansatz, which conveniently does not impose any ad- ditional computational costs. After the successful initial assessment, we developed more efficient implementations of these methods by employing the local approaches based on pair natural orbitals. This way, we extended the range of possible applications to larger systems with thousands of basis functions. To assess the accuracy of TCC as well as its local counterparts, we performed a variety of benchmark calculations ranging from small, yet challenging systems such as the nitrogen molecule or tetramethyleneethane diradical, to larger molecules like oxo-Mn(Salen) or Fe(II)-porphyrin model. 1
Quantum mechanical study of the electron hoping processes of conjugated systems.
Fatková, Kateřina ; Pospíšil, Miroslav (advisor) ; Pittner, Jiří (referee)
This thesis uses previously proposed methodology for simulations of all-trans- polyenes with conjugated systems. Dynamic properties, especially the mean lifeti- mes of the excited states, of these molecules were systematically simulated. Obta- ined data shows that the method is still too time-consuming for polyene molecules with more than 20 carbon atoms, including most carotenoids. Thus, a study of active space reduction was performed with the model tetradecaheptaene molecule with regards to excited state mean lifetimes. A new, less time-consuming method would need further simulation studies. Moreover, static spectra of the these mo- lecules were studied as well, yielding a comparison of different DFT and ab-initio approaches. 1
Multireference coupled cluster methods with local pair natural orbital approach
Lang, Jakub ; Pittner, Jiří (advisor) ; Neogrady, Pavel (referee) ; Musial, Monika (referee)
Multireference coupled cluster (MRCC) methods are a highly accurate approach for sys- tems with quasi-degeneracies, where the static correlation plays an important role. How- ever, while canonical MRCC is successful for many systems, it can be used only for small sized systems. Nonetheless, it was shown that large systems can be described by the domain-based local pair natural orbital approach (DLPNO). In our group, we developed DLPNO-MkCCSD, DLPNO-TCCSD and DLPNO-MkCCSD(T) methods, which were able to recover more than 99.7% of the canonical correlation energy, while the computation of systems with more than 2000 basis functions took only a few hours on a single CPU core. Moreover, we also implemented a tailored variant of MRCC which successfully described excited states of cyclobutadiene, while the traditional MRCC under-performed.
Ab initio molecular dynamics with non-adiabatic and spin-orbit effects applied to time-dependent fluorescence
Pederzoli, Marek ; Pittner, Jiří (advisor) ; Mitric, Roland (referee) ; Gonzáles, Leticia (referee)
Ab initio molecular dynamics with non-adiabatic and spin-orbit effects applied to time-dependent fluorescence Marek Pederzoli1,2 1Charles University in Prague, Faculty of Science, Department of Physical and Macromolecular Chemistry, Hlavova 8, 12840 Prague 2, Czech Republic 2J. Heyrovský Institute of Physical Chemistry, Czech Academy of Sciences, Dolejškova 3, 18223 Prague 8, Czech Republic Fluorescent probes are essential for many experimental techniques in biochemistry and microbiology. Accurate simulations of these molecules are theoretically challenging as they can involve conical intersections and intersystem crossings as well as interactions with the environment. This thesis is a compilation of papers dealing with development, implementation, and application of ab initio molecular dynamics techniques with non-adiabatic and spin-orbit interactions that can be used to model fluorescent probes not only in the gas phase but also in the complex molecular environment of biomembranes. Initial work involves a study of 9H-adenine using ab initio MD with non-adiabatic ef- fect using time-dependent density functional theory and two single reference methods CC2 and ADC(2). The central part of the thesis discusses the implementation of spin-orbit coupling into surface hopping dynamics with application to...
Optimizing quantum simulations and the DMRG method
Brandejs, Jan ; Pittner, Jiří (advisor) ; Zamastil, Jaroslav (referee)
Title: Optimizing quantum simulations and the DMRG method Author: Jan Brandejs Department: Department of Chemical Physics and Optics Supervisor: doc. Dr. rer. nat. Jiří Pittner, DSc., J. Heyrovský Institute of Physical Chemistry of the Czech Academy of Sciences Abstract: In this work, we explore the quantum information theoretical aspects of simulation of quantum systems on classical computers, in particular the many- electron strongly correlated wave functions. We describe a way how to reduce the amount of data required for storing the wavefunction by a lossy compression of quantum information. For this purpose, we describe the measures of quantum entanglement for the density matrix renormalization group method. We imple- ment the computation of multi-site generalization of mutual information within the DMRG method and investigate entanglement patterns of strongly correlated chemical systems. We present several ways how to optimize the ground state calculation in the DMRG method. The theoretical conclusions are supported by numerical simulations of the diborane molecule, exhibiting chemically interest- ing electronic structure, like the 3-centered 2-electron bonds. In the theoretical part, we give a brief introduction to the principles of the DMRG method. Then we explain the quantum informational...
Density matrix renormalization group calculations and entanglement optimization in quantum chemistry
Antalík, Andrej ; Pittner, Jiří (advisor) ; Chalupský, Jaromír (referee)
Density matrix renormalization group is a powerful numerical approach originating in solid state physics. Since its introduction to quantum chemis- try it has been successfully applied to many challenging problems and proved especially efficient for systems with strong multi-reference character. As the method is not invariant to the change of orbital ordering, we implemented the automatic orbital ordering optimization procedure built around the entan- glement analysis and employing the Fiedler method. A technique for active space selection based on one-orbital entropy is introduced. We followed the study by Berardo et al. in which EOM-CC methods were applied to excited states of TiO2 clusters. Four lowest-lying singlet excited states of monomer, dimer and trimer were calculated by means of the DMRG method. In case of the monomer there was good agreement for two states, but the calculations suffered from insufficient treatment of dynamic correlation. For the dimer our results were in very good agreement with EOM-CCSDT results and for the trimer further study would be appropriate. 1
Quantum computers, principles and latest development
Ješko, Eduard ; Skála, Lubomír (advisor) ; Pittner, Jiří (referee)
The main goal of this thesis is to give information on quantum computer principles and its latest development. In the first part we introduce quantum bits, quantum registers and quantum gates. We show basic operations acting on one and more qubits. On an example we present, how it is possible to construct an arbitrary gate using only elementary quantum gates. We describe a behaviour of quantum computers called quantum parallelism and show its application in Deutsch's algorithm. We define the quantum Fourier transform and its applications. In the last chapter we explain on what principle the D-Wave quantum computer works. In this thesis we compare classical and quantum computers in terms of algorithms and computational speed.
Explicitly Correlated Multireference Coupled-Cluster Study of Molecules and Cations with Diradical Character
Jungwirth, Jakub ; Pittner, Jiří (advisor) ; Fišer, Jiří (referee)
Known and proposed helium chemistry embraces a large variety of small-size cationic species. Even though some of these species can be studied experimentally in the gas phase, the vast majority of contemporary studies are backed up or done entirely using high-level of theory calculations. The aim of this thesis is to present a theoretical introduction to ab initio computational chemistry from the basics (definition of the electronic problem, Born-Oppenheimer approximation, Hartree- Fock method) to the Brillouin-Wigner and Mukherjee Hilbert-space state-specific multireference coupled-cluster (MRCC) methods and their explicitly correlated versions. The thesis also contains a study of HeN2+ 2 using the mentioned MRCC methods. An important result has been, that Mukherjee's state-specific multireference coupled-cluster method with perturbative triexcitations Mk CCSD(Tu) gives results in very good agreement to conventional calculations (CASPT2 and MRCI) using a much smaller active space and smaller basis sets.
Aplication of explicitly correlated multi-reference coupled cluster methods
Lang, Jakub ; Pittner, Jiří (advisor) ; Fišer, Jiří (referee)
Nowdays, coupled cluster method belongs to one of the most used quantum chemical methods. However, the single-reference coupled cluster methods are not able to describe systems where the static correlation have an important role. Multireference coupled cluster methods developed in our group can describe both static and dy- namic correlation and can be used for problematic systems. Together with explicitly correlated wavefunction, which can properly describe the electronic cusp and speed up the convergence to the complete ba- sis set limit, they are able to calculate computationally demanding diradicals. Multireference CC calculations of tetramethylenethane have been perforemd and the performance of explicitly correlated version is discussed. Calculations of the isomerization of bicyclobu- tane using the multireference approach are presented as well. 1

National Repository of Grey Literature : 25 records found   1 - 10nextend  jump to record:
Interested in being notified about new results for this query?
Subscribe to the RSS feed.