Browsing by Author "Andrzejak, Marcin"

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Evolution of physical properties of conjugated systems
(2012-02-01)
Li, Wun-Fan
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Andrzejak, Marcin
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Witek, Henryk A.
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應用化學系
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應用化學系分子科學碩博班
Evolution of various physical properties of finite conjugated systems with increasing molecular size is studied using the self-consistent-charge density-functional tight-binding (SCC-DFTB) method. We usually observe a transition from the finite to the infinite regime for surprisingly short molecular chains containing only 20-30 monomer units of cis-butadiene, cyclopentadiene, pyrrole, furan, and thiophene. Such a fast convergence of equilibrium geometries, induced atomic charges, electron densities of states (DOS) and energy gaps, dipole and quadrupole moments, and polarizabilities is particularly striking considering the presence of strong pi-conjugation.
Excited statesmanifold of 2,2 '-bithiophene: basis set dependence study
(2017-01-01)
Andrzejak, Marcin
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Kukulka, Mercedes
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Witek, Henryk A.
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應用化學系
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應用化學系分子科學碩博班
The 2,2'-bithiophene energy spectra computed using a sequence of cc-pVnZ and aug-cc-pVnZ basis sets reveal surprisingly strong dependence of the excited states manifold on the quality of the basis set. The observed computational artefacts include: numerous missing states, wrong order of states and considerable shifts in the energy spectrum. The presented results suggest that the cc-pVnZbasis sets are completely unsuitable for modelling optical spectra of organic molecules and that the augcc- pVnZ basis sets are capable of predicting only the lowest portion of the energy spectrum. A simple and inexpensive remedy for the observed problems is suggested: an additional, molecule-centred, Rydberg basis should be rudimentarily used in quantum chemical calculations aiming at modelling optical spectra of molecules. The main conclusions of the presented study seem to be general and independent of the chemical identity of the studied system and of details of the employed computational methodology. [GRAPHICS] .
The elusive excited states of bithiophene: a CASPT2 detective story
(2011-05-01)
Andrzejak, Marcin
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Witek, Henryk A.
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應用化學系
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應用化學系分子科學碩博班
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Department of Applied Chemistry
A systematic multi-reference perturbation theory investigation of the excitation energies and oscillator strengths for the lowest excited states of 2,2'-bithiophene unequivocally shows that its optical spectrum is produced by two (1) B (u) states separated from each other by approximately 1 eV. This picture is confirmed by additional calculations with alternative quantum chemical methods. Our findings are in strong contrast with the previous CASPT2 results of Rubio et al. [J Chem Phys 102:3580 (1995) and Chem Phys Chem 4:1308 (2003)], who predicted that the two lowest (1) B (u) states are quasi-degenerate. The methodological reasons responsible for the previous seemingly erroneous assignment of the optical spectrum of bithiophene are identified and explained in terms of unusually large coupling between the (1) B (u) states introduced by dynamical correlation effects. A general discussion of applicable computational techniques is offered aiming at avoiding similar problems for other molecular systems.