In this section, you will learn about how to build up molecular orbitals.

1. Use the guest account to log in the WebMO demo site.
Click New Job -> Open Editor. A small window opens where you build molecules.

2. Add C atom in the workspace.
Click on the Periodic Table icon (5th down the left side).? Choose ¡§C¡¨ by clicking on it. Click once in the center of the workspace. A gray carbon atom appears. Click on this atom and drag to one side and release. You should now have C-C.
Click and Drag from the center of one C atom to the center of the other C atom to form the double bond.

3. Choose Clean-Up -> Comprehensive Mechanics. You should now have ethene, CH2CH2. Click Close Editor in the main WebMO window.

4. Click the blue continue arrow in the lower right side of the Build Molecule window. Choose Mopac as the computational engine and click the blue continue arrow. Use Job Name C2H4PM3, and perform the geometry optimization as before (PM3). Once the job is complete, click the hyperlinked name (C2H4PM3) to open the View Job window.

5. Click the New Job Using This Geometry button, click the blue continue arrow, choose Mopac, and type in/Choose the following: Job Name: C2H4 PM3 MO, Calculation: Molecular orbitals, Theory: PM3, Charge: 0, Multiplicity: Singlet. Click on the blue continue arrow. You should now see your job listed. Once the job is complete, click on the hyperlinked name (C2H4 PM3 MO) to open the View Job window.

6. Scroll down to the Molecular Orbitals table. Using the occupancy values as your judge, identify both the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO). View these orbitals by clicking on the magnifying glass icon. The orbitals can be rotated by clicking and dragging in the viewer window. If you hold down the Shift key while clicking and dragging, you can zoom in and out. The HOMO is the pi orbital between the carbon atoms. The LUMO is the anitbonding pi* orbital View some of the other orbitals. Note that all orbitals viewed appear as a tab in the MOViewer. Clicking on the tab brings that orbital back into the window. Close the viewer window and return to Job Manager.

7. Note that all orbitals viewed appear as a tab in the MOViewer. ?Clicking on the tab brings that ? orbital back into the window.? Close the viewer window and return to Job Manager. To explore more about bonding and antibonding orbitals, see the Web site: http://winter.group.shef.ac.uk/orbitron/ Scroll down to the N2, Dinitrogen section to observe formation of pi and pi* type molecular orbitals.