Bulk potential energy by different methods
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
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