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4.4 Chapter 4 conclusions
A series of calculations of energies and oscillator strengths of transitions to electroni-cally
excited states of uracil show that using high-quality basis sets (i.e., aug-cc-pVTZ
or aug-ANO-DZ) with diffuse functions is essential for converged results.
Active-space EOM-CC calculations with explicit triple excitations demonstrate that
the excited states do not exhibit a doubly excited character, and therefore EOM-CCSD
provides a good zero-order wave function for including remaining dynamical correlation
via perturbative treatment of triple excitations. The latter affects excitation energies by
as much as 0.3 eV. Thus, the EOM-CCSD errors for the excitation energies of uracil do
not exceed the conventional estimates of the EOM-EE-CCSD error bars.
Our best estimate of excitation energies calculated with CR-EOM-CCSD(T)/aug-cc-pVTZ
places the np singlet excited state at E
11A00
=5.00 eV, and the pp singlet state
at E
11A0
=5.25 eV above the ground state. An excellent agreement between high-level
MRCI calculations, which place the pp state at 5.32 eV, and EOM-CC further supports
our conclusions. A solvent (water) affects the excitation energies of the two states by
+0.5 eV and less than 0.1 eV, respectively.
We conclude that the maximum of the broad UV spectrum of uracil does not corre-spond
to the position of the vertical pp excitation, possibly due to strong vibronic cou-pling
with the lower-lying dark np state. Our results indicate that previously reported
CASPT2 and TDDFT calculations underestimate the excitation energies.
128
Object Description
| Title | Development of predictive electronic structure methods and their application to atmospheric chemistry, combustion, and biologically relevant systems |
| Author | Epifanovskiy, Evgeny |
| Author email | epifanov@usc.edu; epifanov@usc.edu |
| Degree | Doctor of Philosophy |
| Document type | Dissertation |
| Degree program | Chemistry |
| School | College of Letters, Arts and Sciences |
| Date defended/completed | 2011-03-21 |
| Date submitted | 2011 |
| Restricted until | Unrestricted |
| Date published | 2011-04-28 |
| Advisor (committee chair) | Krylov, Anna I. |
| Advisor (committee member) |
Wittig, Curt Johnson, Clifford |
| Abstract | This work demonstrates electronic structure techniques that enable predictive modeling of the properties of biologically relevant species. Chapters 2 and 3 present studies of the electronically excited and detached states of the chromophore of the green fluorescent protein, the mechanism of its cis-trans isomerization, and the effect of oxidation. The bright excited ππ∗ state of the chromophore in the gas phase located at 2.6 eV is found to have an autoionizing resonance nature as it lies above the electron detachment level at 2.4 eV. The calculation of the barrier for the ground-state cis-trans isomerization of the chromophore yields 14.8 kcal/mol, which agrees with an experimental value of 15.4 kcal/mol; the electronic correlation and solvent stabilization are shown to have an important effect. In Chapter 3, a one-photon two-electron mechanism is proposed to explain the experimentally observed oxidative reddening of the chromophore. Chapter 4 considers the excited states of uracil. It demonstrates the role of the one-electron basis set and triples excitations in obtaining the converged values of the excitation energies of the nπ∗ and ππ∗ states. The effects of the solvent and protein environment are included in some of the models.; Chapter 5 describes an implementation of the algorithm for locating and exploring intersection seams between potential energy surfaces. The theory is illustrated with examples from atmospheric and combustion chemistry. |
| Keyword | electronic structure theory; coupled clusters theory; equation of motion theory; organic chromophore; green fluorescent protein; uracil |
| Language | English |
| Part of collection | University of Southern California dissertations and theses |
| Publisher (of the original version) | University of Southern California |
| Place of publication (of the original version) | Los Angeles, California |
| Publisher (of the digital version) | University of Southern California. Libraries |
| Provenance | Electronically uploaded by the author |
| Type | texts |
| Legacy record ID | usctheses-m3801 |
| Contributing entity | University of Southern California |
| Rights | Epifanovskiy, Evgeny |
| Repository name | Libraries, University of Southern California |
| Repository address | Los Angeles, California |
| Repository email | cisadmin@lib.usc.edu |
| Filename | etd-Epifanovskiy-4557 |
| Archival file | uscthesesreloadpub_Volume14/etd-Epifanovskiy-4557.pdf |
Description
| Title | Page 138 |
| Contributing entity | University of Southern California |
| Repository email | cisadmin@lib.usc.edu |
| Full text | 4.4 Chapter 4 conclusions A series of calculations of energies and oscillator strengths of transitions to electroni-cally excited states of uracil show that using high-quality basis sets (i.e., aug-cc-pVTZ or aug-ANO-DZ) with diffuse functions is essential for converged results. Active-space EOM-CC calculations with explicit triple excitations demonstrate that the excited states do not exhibit a doubly excited character, and therefore EOM-CCSD provides a good zero-order wave function for including remaining dynamical correlation via perturbative treatment of triple excitations. The latter affects excitation energies by as much as 0.3 eV. Thus, the EOM-CCSD errors for the excitation energies of uracil do not exceed the conventional estimates of the EOM-EE-CCSD error bars. Our best estimate of excitation energies calculated with CR-EOM-CCSD(T)/aug-cc-pVTZ places the np singlet excited state at E 11A00 =5.00 eV, and the pp singlet state at E 11A0 =5.25 eV above the ground state. An excellent agreement between high-level MRCI calculations, which place the pp state at 5.32 eV, and EOM-CC further supports our conclusions. A solvent (water) affects the excitation energies of the two states by +0.5 eV and less than 0.1 eV, respectively. We conclude that the maximum of the broad UV spectrum of uracil does not corre-spond to the position of the vertical pp excitation, possibly due to strong vibronic cou-pling with the lower-lying dark np state. Our results indicate that previously reported CASPT2 and TDDFT calculations underestimate the excitation energies. 128 |
