For properly using GAMESS you need two more programs. One to create the input and one for looking at the output file. Ghemical-GMS works as a front end program. But you can also type the input file into a text editor. That's what I did because extracting the .zip did not work. I downloaded Chemcraft for looking at the results. MacMolPlt is probably also a good program but I haven't tried it out.
For my first example I optimised the geometry of iron-pentacarbonyl. The input file looks like this (spaces before $ are important):
! Geometry optimisation for iron-pentacarbonyl.
$CONTRL SCFTYP=RHF RUNTYP=OPTIMIZE $END
$SYSTEM TIMLIM=30 $END
$BASIS GBASIS=N21 NGAUSS=3 $END
$GUESS GUESS=HUCKEL $END
C 6 1.5
O 8 2.5
C 6 0 0 1.5
O 8 0 0 2.5
After a short comment comes the $CONTRL group. In there I am saying that I want restricted hartree fock which works for closed shell molecules and that I want to optimize the geometry. Next I set the time limit to 30 minutes. Next I am saying that I want to use Pople's 3-21G basis set (because that sounded nice, information may be found here). Then I am saying that I want the program to guess the initial orbitals of the SCF run according to the extended Hückel theory.
The actual molecule is in $DATA. First an arbitrary name. Next the symmetry group D3h. Then comes the molecule data. I only have to put symmetry unique atoms here. Iron with nuclear charge 26 at the origin (I could write 0 0 0 here). Then an equatorial C and O along the x-axes 1.5 and 2.5 angstroms away from the center. Two more CO sets are added because of the symmetry group. Then I add the axial C and O along the z-axis (by default the main axis of rotation). The other one is added automatically.
Next I create the jobs.bat file in batmaker.exe. If I click this file, the job is executed. The result can be visualised in ChemCraft.
This is what the geometry optimisation looked like. Every second step is shown. The optimisation was fast even though the molecule is rather big because of symmetry. There are only 4 parameters that can be changed, the equatorial and axial Fe-C and C-O distances.
With GAMESS and ChemCraft you can produce nice looking MO graphics. I chose most of the totally symmetrical A'1 MOs.