Quantum Calculations

David Abbott and Sarah Schettle

Experiment dates:  6-22 February 2007

Introduction
GAMESS software is capable of running MOPAC semi-empirical and ab initio quantum calculations on a MacOS X computer.  These calculations allowed us to examine three molecules: N2, HCl, and ortho-dibromobenzene in depth.  Applying different levels of theory to nitrogen, hydrogen chloride, and ortho-dibromobenzene gives varying degrees of geometry for the molecule.  Choosing the method with the best optimized geometry allows for analysis of bond length, bond order, and dipole moment of the molecule.  Detailed figures of optimized geometry and molecular orbitals (HOMO and LUMO) accompany these results.  Potential energy spectra show energy wells with the best geometry indicated by the lowest energy well.  UV-Vis data indicate the absorbance frequencies from the ground to various excited states.  Analysis of the results and figures can allow for a better understanding of these molecules.

Click on these fabulous links to learn fascinating information about these molecules.


HCl

N2

o-Dibromobenzene



Conclusions
Computational results are useful for comparing bond stretches and vibrations in molecules. Computer calculations clearly show which orbitals are occupied, unoccupied, and which are involved in bonding. Different levels of theory produce results of varying accuracy. The level of accuracy corresponds to the size of the basis set used in computations. In general, we found that the highest basis sets resulted in calculated data and molecular geometry that came closest to matching experimental data. Calculated data approached accepted literature values as the basis sets increased.


Website Authored 1 March 2007
Copyright 2007 David Abbott and Sarah Schettle
Last Updated 6 March 2007