Molecular Calculations of Diatomic Oxygen, Formaldehyde, and Benzaldehyde

    Molecules will be in the geometry that have the lowest energy.  These geometries can be calculated using advanced computer modeling programs.  "GAMESS" is one such program.  In GAMESS, there are options to perform calculations as MOPAC or ab initio.  MOPAC calculations are based off semi empirical calculations of electron overlap intergrals needed to calculate the Hamiltonian. Ab initio calculations are based off intergral calculations.  Ab initio calculations are more in depth and often yield more accurate calculations.  One draw back of ab initio calculations is that they often strain the computer in which the calculations are being made.  This slows down the computer and often takes several hours to complete. 
    After the optimized geometries of molecules have been obtained, other computer programs can be used to model other aspects of the molecules.  The HOMO, vibrational data, and the potential of bond stretching can all be modeled from various programs.  These calculations can then be related to scientifically accepted values.  Often times, the calculated values are similar to the accepted values.  This makes sense considering these values are often calculated using computer programs.  The reason for the discrepencies between the accepted values and the calculated values is often due to the depth of calculations performed.  At times, there are large differences between the calculated and accepted values.  
In conclusion, the calculations made by this software were very useful in determing the probable shape of the molecules, electromagnetic configurations, and visualizing sub shells. The calculations for the molecular geometries, bond angles and lengths, agreed reasonably well with those that are scientifically accepted. The vibrational frequencies calculated provided another look into the workings of various molecules.  The calculated vibrational frequencies could then be related to actual vibrations seen on IR spectra.  The "GAMESS" software provided a good starting location.  Even through the software gave a good starting spot for calculations, more precise calculations could be performed to better the results.  The bright spot of using this software was the very good geometries that were obtained.  Even tough these geometries were not completely accurate, they work for most analysis.  As molecules become more complex, drawbacks of this software begin to appear.  This software does would not handle comlex molecules well.  Calcuations for large molecules would take days or even weeks to complete.  The error associated with complex molecules would be large.  This software works by guessing at the geometries of the molecules.  An error in one geometry would lead to a bigger error in the next geometry, because the previous geometry is used as a starting point for the next calculation.  Large molecules could also pose a problem for this software in terms of local energy minima.  Large molecules could have several geometries.  If the software became bogged down in the wrong geometry, the best geometry produced would be for the local minima and not the geometry with the lowest overal energy.