Bond Rotation Energy Profile of 1,2-Dichloroethane

If a dihedral is selected and stored in Build Mode and the bond is not in a ring then the energy profile observed on rotating one end of the bond may be calculated using TinkerMopac or Gamess.

Build 1,2-dichloroethane.

The rotamer produced does not matter. In Build Mode click StoreBond(s) on the menu and then use to select the atoms along the bonds in the order Cl-C-C-Cl and then click and go to the Mopac menu. Click RunMopac and OK to optimize the structure. Click NewInput and

on dialog 10 check Bond Rot.Profile, Energy and click Run Mopac. The program will calculate the energy at each step as the rotation takes place when finished the results are in the GEO file. These may be plotted using Excel. To plot the results click the right mouse button to go back to display mode and click Edit.GEO.

Select the columns of numbers and paste into Excel. In Excel click Data then Text to columns then Fixed width then Finish. Select all three columns and click Data and Sort. The default of Column A ascending is fine. Select the first two columns click the plot button and select XY (Scatter).

Click Finish

There is one large peak which is observed at 0 where the Cl atoms eclipse one another. The results obtained using Tinker show exactly the same profile. The second column contains the energy profile in relative kJ/mol and this is the most useful for plotting.

Bond Rotation Profile with Relaxation (partial optimization)
Using GAMESS it is possible to allow the structure to relax at each position along the profile. This is sometimes called partial optimization. To do this select ZMAT and Bond Rot. Profile on the GAMESS dialog. The optimization is performed in internal coordinates, more details are given in the Moilin help. The optimization may fail in more complex cases if you crash groups into one another and make the optimization gradient too steep. To avoid this reduce the step size to 5 deg. The default step size can be changed on Setup.
A partial optimization run output is like the following;

and of the shape of the profile is unchanged