Chemistry Reference
In-Depth Information
10
ACCURACY AND METHODS BEYOND
“STANDARD” CALCULATIONS
10.1 HOW ACCURATE ARE DFT CALCULATIONS?
Throughout this topic, we have focused on
using
plane-wave DFT
calculations. As soon as you start performing these calculations, you should
become curious about how accurate these calculations are. After all, the aim
of doing electronic structure calculations is to predict material properties
that can be compared with experimental measurements that may or may not
have already been performed. Any time you present results from DFT
calculations in any kind of scientific venue, you should expect someone in
the audience to ask “How accurate are DFT calculations?”
So how accurate are DFT calculations? It is extremely important to recog-
nize that despite the apparent simplicity of this question, it is not well posed.
The notion of accuracy includes multiple ideas that need to be considered sep-
arately. In particular, it is useful to distinguish between
physical accuracy
and
numerical accuracy
. When discussing physical accuracy, we aim to under-
stand how precise the predictions of a DFT calculation for a specific physical
property are relative to the true value of that property as it would be measured
in a (often hypothetical) perfect experimental measurement. In contrast,
numerical accuracy assesses whether a calculation provides a well-converged
numerical solution to the mathematical problem defined by the Kohn-Sham
(KS) equations. If you perform DFT calculations, much of your day-to-day
