Chapter 3
Heats of Formation of the
Solids
3.1 Introduction
In the last chapter a brief description about the density functional theory
(DFT) was provided with a short introduction to the different functionals and
their accuracy. In this chapter, one of the application of DFT is looked at
i.e. the calculation of heats of formation of the solid compounds with different
functionals particularly focussing on the accuracy of their predictions.
The accuracy of the energetics of a thermodynamic process obtained with
the different functionals depends on the fortuituous cancellation of errors.
However, if the nature of species on the different side of a reaction differs
significantly then the cancellation of errors may not be complete thus leading
to an inaccurate energetics. For example, one of the most basic reaction is the
formation of the solids from the elements in their reference state. In this case
the chemical environment of the solid formed is very different from the chemical
environment of the elemental phases. In cases like these the cancellation
of errors may not be complete thus ending up giving inaccurate results 24.
The same reason renders standard LDA/GGA to give the heat of formation of
the solids deviating from experiments by 0.25 eV per atom 25. Therefore,
large errors in the prediction of the heats of formation may not be appropriate
in situations like large scale screening of materials where thermodynamic stability
is one of the main criterion for the existence of the compounds 26, 27.
Hence, in order to get greater accuracy higher level methods are required. On
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