Computational Chemistry Comparison and Benchmark DataBase Release 22 (May 2022) Standard Reference Database 101 National Institute of Standards and Technology
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You are here: Resources > Tutorials > Energy > Getting enthalpy of formation

How to get an enthalpy of formation from ab initio calculations

1. Choose a reaction with experimentally known enthaplies of formation for all species except the one of interest.
   1. example
   2. why do we need a whole reaction?
   3. what makes a good reaction?
2. Calculate the energies for all species in the reaction at the same level of theory.
   1. What level of theory should I use?
   2. Optimize the geometries.
   3. Calculate vibrational frequencies. (Why do we need the vibrational frequencies?)
   4. (Optional) Calculate the electronic energy for all optimized geometries at some higher level of theory.
3. (Optional) Convert from 0K to 298.15K (or some other temperature).
4. Combine the various energies to get a reaction energy.
   1. For each species we want the electronic energy plus the vibrational zero point energy.
   2. The reaction energy is the sum of the product energies minus the sum of the reactant energies.

In all the above examples we have used experimental enthalpies of formation reported at 0 K (zero Kelvin). Usually it is more useful to predict enthalpies of formation at room temperature (298.15 K) or some other temperature. To do this we need to convert our 0 K enthalpy of formation to an enthalpy of formation at our desired temperature. This is described on the next page.