Bulk potential energy by different methods

Sebastian Hütter sebastia... at ovgu.de
Fri Jun 2 10:25:48 UTC 2017

Hi Matt, Matthias,

thank you for your replies.

The input file is basically identical to a similar question about Nickel 
posted here some time ago 
<https://groups.google.com/d/msg/cp2k/llUH48_Y3vY/GPrPXaaXZJIJ>. I did 
convergence studies on EPS_SCF and cell replication count NREP; the 
parameters posted are the fastest in terms of simulation time. For the Evf 
calculations, I used 5x5x5 unit cells (500/499 atoms) for both methods.

individual total energy are almost always meaningless, especially when 
comparing different levels of theory.
I'm still new to this, but... why? Shouldn't the total energy be a direct 
consequence of the cohesive energy, especially in such simple systems?

a gamma point calculation for Al metal using a model system consisting of 
only 3x3x3 unit cells won't provide converged results. You need a much 
larger model system in the order of 10x10x10 unit cells or you have to 
employ an equivalent k point sampling. Besides that major issue, there are 
further shortcomings in your CP2K input: (1) an SCF convergence threshold 
of 1.0E-4 (EPS_SCF) is too large: 1.0E-6 (or smaller), (2) a general 
threshold of 1.0E-10 (EPS_DEFAULT) is also too large: 1.0E-12 (or smaller)
I have redone the calculation with your suggested thresholds and 6x6x6 unit 
cells which is about the largest I can run on our local compute server 
because of memory constraints, if that showed any change at all I would 
have to post to the HPC cluster. However: no change. Still -8.38Ha/UC.

Is LDA a good enough density functional. Is a DZVP basis set good enough?
I don't know? How do I find out?

Did you relax the structures?
A CELL_OPT run changed the total energy by just a few percent (for this 
setup, a=4.05 is a bit on the high side), not the order of magnitude that's 
missing here.

Kind regards,

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