What's the difference -- MOPAC

Task: Calculate the Heat of formation of ammonia using MOPAC (AM1 and PM5)

Time: 4 minutes

The images in this section of the tutorial use ammonia  built in previous sessions.

Important precautions: The objective of this series of exercises is to compare the heat of formation computed by three different methods.  Therefore it is essential begin with the same file each time and save it with a new name before beginning the calculation.

1. Use the ammonia molecule built previously: File|Open "ammonia". Skip to step 3.
2. Or, build ammonia from scratch
   1. Open a new workspace: File|New
   2. Using the Atom icon, select from the element type box, make sure the hybridization is and click in the workspace to draw a nitrogen atom .
   3. Add the hydrogens and fix the geometry by selecting Beautify|Comprehensive.
3. Compute the enthalpy of formation of ammonia using AM1

1. Save the file, File|Save As "ammonia-AM1".
2. Select Experiment|New
3. In the Property of: box, select chemical sample.
4. in the Property box, select optimized geometry.
5. in the Using box select AM1 geometry.
6. Click Start.
4. Record the value of the heat of formation of the molecule found in the experiment status box.
5. Compute the enthalpy of formation of ammonia using PM5
   1. Open the molecule of ammonia, File|Open "ammonia".
   2. Save the file, File|Save As "ammonia-PM5".
   3. Repeat steps 3c and 3d above.
   4. In the Using box select PM5 geometry.
   5. Click Start.
6. Record the value of the heat of formation found in the experiment status box.

Compute the enthalpy of formation of ammonia using DFT in the next part of this exercise.