P&A tasks

The video mentioned a website with a database of Δ-values. Go there, and inspect the list. Locate your favourite DFT code/method, and read its global Δ-value. What do you conclude from this Δ?

From this same website, you can make a legal download of the paper Reproducibility in density functional theory calculations of solids that was mentioned in the video. From Fig. 4 in the paper, you read that the Elk code has a Δ of 0.3 meV/atom with respect to the WIEN2k code. Verify that in the table on the website, which is ordered by default with respect tot WIEN2k (on top). Then click on the line with Elk, after which the table gets reordered with Elk on top. Verify now that in agreement with Fig. 4 the Δ of the RSPt code with respect to Elk is 0.9 meV/atom.

Now click on the Δ-value for Elk (the 0.9), and a periodic table pops up with the individual values for every element. Which element contributes most to the global Δ between Elk and RSPt?

This was not the first time we met the concept of testing the precision of a DFT calculation. Actually, you have done this already several times yourself (although we didn’t always call it by that name). Explain when you have been doing precision testing (in the hands-on part or for the project). Make clear why that was precision testing, and not accuracy testing.

You meet a friend who studies at a richt foreign university, with access to computing resources that are 50 times better than what you have. You both have the same DFT code, and you both study the same crystal. She claims that thanks to her faster computer, her results will be closer to the experimental values. What would you reply?

In the video, a paper on accuracy assessment was discussed (open access here). Have a look at the abstract and conclusions, and at the examples in Tab. 5. There was a similar example in the video (about β-Sn). Now take the DFT(PBE) prediction for the equilibrium volume of litharge that were made a few weeks ago in the hands-on part of the course. Estimate quantitatively the accuracy of this DFT prediction, following the method from Tab. 5 or from the video. Compare this with the experimental value. What do you conclude? Was this to be expected?