This dissertation describes a tight-binding technique that treats the dynamics
of electrons and ions simultaneously. The main features are a generalized Hellmann-
Feynman theorem, a standard, time-dependent, self-consistent-field description and
the interaction picture. The time-dependence is incorporated by using Peierls Substitution.
We also apply the velocity-Verlet algorithm to predict the motion of the
ions.
We first test the validity of this semi-empirical tight-binding approach on several
smaller systems including ethylene, 2-butene, and stilbene. The cis-trans isomerization
is modeled and in each case the results agree well with those obtained from
other computational and empirical methods. Next, we use the tight-binding model
to simulate the photoisomerization of the retinal molecule from its cis to trans form.
The results are comparable to those obtained from experiments. The vibrational frequencies
for retinal obtained using the force-constant techniques in this model agree
well with those obtained from Fourier transform methods and a standard software.
The cis-trans isomerization takes 217.91 fs to complete with a field strength of 1.0
gauss·cm, which is comparable to 200 fs reported from experiments. The isomerization
depends on the strength of the vector potential, the time-step of the simulation and also the wavelength of the light. Using different parameters the isomerization
takes place in 1-2 ps which is within the range reported from experimentation.
The present semi-empirical technique provides an excellent compromise between
computationally-prohibitive first principles methods and approximate empirical methods
to model the motion of electrons and ions in a large molecule like retinal.
| Identifer | oai:union.ndltd.org:tamu.edu/oai:repository.tamu.edu:1969.1/ETD-TAMU-2575 |
| Date | 15 May 2009 |
| Creators | Sinha, Indrani |
| Contributors | Ford, A. Lewis, Lucchese, Robert R. |
| Source Sets | Texas A and M University |
| Language | en_US |
| Detected Language | English |
| Type | Book, Thesis, Electronic Dissertation, text |
| Format | electronic, application/pdf, born digital |
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