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University of Pittsburgh at Johnstown

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What's the difference -- DFT

Task: Calculate the Heat of formation of ammonia using B88-LYP DFT

Time: 3 minutes

Density Functional Theory (DFT) is an ab-initio method.  CAChe uses the DGauss program for DFT calculations.  DGauss has several widely used density functionals and basis sets.  We'll use the B88-LYP hybrid functional with the default basis set.

In the last exercise you probably noticed that we  didn't have to optimize the molecular geometry in order to compute the heat of formation -- or so it seemed.  Actually, MOPAC computes the heat of formation of a molecule as part of the geometry optimization routine.  Calculating thermodynamic properties using DFT is a two-step process. 

First we have to optimize the molecular geometry. Then we do the thermodynamic calculations.  This one will take a little longer.

Important precautions: Begin with a file that has not been optimized. Save the file with a name that reflects the method used.

  1. Optimize the geometry of ammonia using B88-LYP
    1. Open the molecule of ammonia, File|Open "ammonia".
    2. Save the file, File|Save As "ammonia-B88-LYP".
    3. Select Experiment|New.
    4. In the Property of: box, select chemical sample.
    5. in the Property box, select optimized geometry.
    6. In the Using box select B88-LYP DFT geometry.
    7. Click Start.
  2. Calculate the thermodynamic properties of ammonia:
    1. Select Experiment|New
    2. In the Property of: box, select chemical sample.
    3. In the Property box, select IR transitions.
    4. in the Using box select B88-LYP DFT IR spectrum.
    5. Click Start.
  3. Record the heat of formation
    1. Use the scroll bar on the experiment status box and scroll up to find the enthalpy at 298K (in au).
    2. The unit given for the calculated value is in Hartrees (the Atomic unit of Energy, au). The other values are in kcal/mol. 
    3. We have to covert: 1 kcal/mol =  627.51 au
    4. Record this value.
 

In the last part of this exercise we'll compare our computed values with the experimental enthalpy of formation of ammonia.