Bulk potential energy by different methods

Matt W mattwa... at gmail.com
Fri Jun 2 10:45:15 UTC 2017

How do you do the vacancy calculation? Are you still using multiple unit 
cell and just removing an atom?


On Friday, June 2, 2017 at 11:25:48 AM UTC+1, Sebastian Hütter wrote:
> 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,
> Sebastian
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