Publikationsserver der RWTH Aachen University
Excited states and transition metal compounds with quantum Monte Carlo
Abstract
dc:descriptionTo the most challenging electron structure calculations belong weak interactions, excited state calculations, transition metals and properties. In this work the performance of variational (VMC) and fixed-node diffusion quantum Monte Carlo (FN-DMC) is tested for challenging electron structure problems using the quantum Monte Carlo amolqc code by Lüchow et al. The transition metal compounds under consideration are vanadium oxides. Here excitation, ionization, oxygen atom and molecule abstraction, and atomization energies have been studied for the vanadium oxide clusters VOn{+/0} with n=0-4. The reaction energy of V2O5 -> VO3+VO2 was calculated. The complete FN-DMC procedure established includes geometry optimization and calculation of zero point corrections using BP86/TZVP, single point calculation of the BP86/SB type, and optimization of Jastrow parameters in the framework of VMC variance minimization to obtain a suitable guide wave function. A careful adjustment of the pseudopotential evaluation and of the time steps was done to obtain reliable FN-DMC results that proved at least as accurate as results from CCSD(T)/cc-pVTZ calculations including scalar relativistic corrections. This FN-DMC procedure will easily be extendible to larger systems. For the oxygen abstraction and the atomization where experimental data is available for comparison, FN-DMC/BP86/SB always renders the best results of all calculations performed. The dissociation of VO, its vertical ionization and the oxygen abstraction from VO2+ are obtained in excellent agreement to experiment using FN-DMC/BP86/SB. Rydberg excitation energies and singlet triplet splittings are calculated for the carbon atom and carbon monoxide. The considered excitations were from the 3P ground state into the 3P and 1P 2pns (n=3-6) Rydberg states and from 1Sigma into 1Sigma and 3Sigma 5sigma m sigma (m=6-7), respectively. The wave functions used are described in terms of configuration state functions from OSLHF orbitals which are particularly well-suited for the construction of QMC guide and trial functions for Rydberg states. The OSLHF excitation energies are improved with VMC and FN-DMC, respectively. However, fixed-node DMC does not describe the singlet triplet splittings reliably whereas VMC results are in excellent agreement with the experiment. Regional analyses and the newly established weighted FN-DMC approach were able to systematically improve also the FN-DMC singlet triplet splittings over OSLHF and the novel technique can be equally used for the treatment of other excited state systems.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2007
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Bande, Annika
- Contributors dc:contributor
-
- Lüchow, Arne
Subjects
dc:subject × 17Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- eng