John E. Boyd

Date of Award


Document Type


Degree Name

Master of Science


Department of Engineering Physics

First Advisor

Larry W. Burggraf, PhD


Previous AFIT research with density functional theory (DFT) has shown itself to be accurate for small SimCn (m,n ≤ 5) clusters at a fraction of the cost of other quantum mechanical methods, but it is only a ground state theory. Time dependent density functional theory (TDDFT), however, is able to calculate excited states as well. Evaluating the accuracy of these methods with respect to the excited states of these clusters was the focus of this research, specifically with respect to the excitation energies, geometries, and vibrational frequencies. It is shown that for the excited states that can be expressed as a single electron configuration, energies calculated are generally within .1 eV or better of experimental differences. A possible scheme for correcting multiconfigurational states is also presented, which also brings those energies to within .1 eV of experiment. This research has demonstrated the ability of TDDFT to give an accurate picture of silicon carbide excitations, placing future calculations with larger clusters on solid ground. Calculations on larger, cage-like structures show excitation energies consistent with spectroscopic measurements of SiC surface defects, suggesting the possibility that the SiC surface forms similar clusters. Calculations on the equilibrium geometries and vibrational frequencies of yet unobserved states of the smaller clusters can aid in their detection in interstellar atmospheres and the laboratory. Most importantly, this research offers further insight into how silicon and carbon interact with one another, which may one day lead to better semiconductors for aerospace applications.

AFIT Designator


DTIC Accession Number