Bulk potential energy by different methods

[Matthias Krack]

Hi Sebastian

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)

Best regards

Matthias

On Thursday, 1 June 2017 17:31:36 UTC+2, Sebastian Hütter wrote:
>
> Hi all,
>
> this may be a simple question, but somehow I feel like I am missing 
> something very basic here.
>
> I'm simply comparing bulk energies of metals (here: Al) computed over 
> XYZ-periodic unit cells obtained via DFT in cp2k and several EAM potentials 
> in LAMMPS. Different published potentials in LAMMPS all agree on an energy 
> of somewhere around -14 eV/unit cell. However, when I run the same 
> computation in cp2k (input file attached), I find energies (printed as the 
> "FORCE_EVAL ( QS ) energy") of -8.3 Ha/unit cell, or -226 eV/unit cell. As 
> this is more than one order of magnitude off, something can't be right here.
> To see if the relative energies are usable, I then tried to calculate 
> vacancy formation energy by comparing the supercell energies with and 
> without vacancies (cf. 
> https://icme.hpc.msstate.edu/mediawiki/index.php/LAMMPS_Vacancy_Formation_Energy) 
> , but that is wrong as well, while the LAMMPS solution agrees reasonably 
> well with experimental data.
>
> It appears as if every energy value calculated is too large by a factor of 
> 16? Why would that be the case?
>
>
> Thank your for your help,
>
> Sebastian
>
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