[Pw_forum] Converged SCF total energies different in 6.0/6.1/6.2

[Paolo Giannozzi]

See this note in file Doc/release-notes:

  * Some constants in the definition of PBE functionals were truncated to
    6 significant digits. While not a bug, this could lead to tiny
differences
    with respect to previous results and other XC implementations (r13592)

More explicitly: some constants that previously were truncated to 6 digits,
are now defined with full precision. This removes minor numerical
discrepancies with the results obtained using libxc.

Paolo

On Thu, Dec 21, 2017 at 3:11 AM, WF <kalamaillist at gmail.com> wrote:

> Dear all,
>
> In tests a normal DFT calculations with Si, I noticed that QE 6.0 and 6.1
> gives exactly same total energy with 6.0 and 6.1 (-15.75190880 Ry), but
> with QE 6.2.1, the total energy is -15.75191309Ry. I tested this on two
> different machines with different compilers, the results are the same
> across all of them (at least to last digit). I did some tests on larger
> systems also and found even larger difference. This 1e-5 Ry order of
> difference is not negligible. Is there anything introduced in 6.2 leads to
> this difference? Thanks very much.
>
>
>
> Best regards,
>
> Feng
>
>
>
> The input file is attached:
>
> ====================
>
> &control
>
>     calculation='scf'
>
>     outdir='./tmp'
>
>     prefix='si'
>
>     wf_collect = .true.,
>
> /
>
> &system
>
>     ibrav=2,
>
>     celldm(1) = 10.263,
>
>     nat=2,
>
>     ntyp= 1,
>
>     ecutwfc = 40,
>
> /
>
> &electrons
>
>     mixing_mode='plain'
>
>     mixing_beta = 0.7,
>
>     conv_thr =  1.0d-10,
>
> /
>
> ATOMIC_SPECIES
>
> Si  1.0 Si_ONCV_PBE-1.1.upf
>
> ATOMIC_POSITIONS {alat}
>
> Si 0.00 0.00 0.00
>
> Si 0.25 0.25 0.25
>
> K_POINTS (automatic)
>
> 4 4 4 0 0 0
>
> ==============
>
>
>
>
>
> --------------------------------------------------
>
> Dr. Feng Wu
>
> Postdoctoral researcher
>
> Department of Chemistry and Biochemistry
>
> University of California, Santa Cruz
>
> Phone: 831-459-2874 <(831)%20459-2874>
>
>
>
>
>
> _______________________________________________
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> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>



-- 
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
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