Computer software is capable of optimizing molecular geometries by calculating energies and choosing the best geometry based on the one with the most negative energy.  Different basis sets are used at different levels of theory.  By optimizing a molecule's geometry, the software can calculate dipole moments, bond lengths, and bond orders.  The software can also calculate vibrational frequencies and UV-visible transitions.  The software is not perfect and will output different answers based on the level of theory and basis set.  The molecules analyzed are Cl2, NO, and bromobenzene.  Calculated values are compared to referenced data from various sources.

    Three molecules were analyzed using molecular modeling as part of the GAMESS-US software package.  The molecules were chlorine (Cl2), nitric oxide (NO), and bromobenzene.  The program Ghemical was used to draw and input the approximate geometry of bromobenzene into the GAMESS software, but the program WebMO was used for the diatomics Cl2 and NO.  Different levels of theory were used to optimize the molecule's geometries and find their dipole moment.  Diagrams of primary molecular orbitals, HOMO and LUMO orbitals, electron density maps, electrostatic potential maps, and partial atomic charges were produced from the software.  The semi empirical method, MOPAC, used hamiltonians with different basis sets; the levels of theory used were AM1 and PM3.  Ab initio is a better level of theory which calculates integrals and uses a Gaussian basis set to find optimized geometries; the levels of theory used were 3-21G and 6-31G.  The best basis set for each molecule was choosen and input into wxMolPlt; the vibrational frequencies were calculated for the diatomics and the UV-visible transitions calculated for bromobenzene.


Cl2      NO      Bromobenzene

    The software was able to produce somewhat good data for the geometries, dipole moments, bond lengths, bond orders, and vibrational frequencies of the Cl₂, NO, and bromobenzene.  The output of the GAMESS software varied from literature values due to the parameters used in the basis sets.