[QE-users] Variation in ph.x results for QE 6.5 and QE 6.4

[Paolo Giannozzi]

Apparently the largest discrepancies come from low-energy modes, in
particular zero-frequency acoustic modes (those with "negative" frequencies
that become zero when acoustic sum rule is applied). Even minor changes in
the way exchange-correlation is computed may lead to surprisingly large but
unphysical differences for those modes.

Paolo

On Sun, Sep 13, 2020 at 7:54 PM Unmesh Mondal <
unmesh.mondal at students.iiserpune.ac.in> wrote:

> Dear all,
> I intend to calculate the vibrational modes of  gas phase HCOOH (formic
> acid) molecule.
>
> The "ph.x" vibrational frequencies (in cm-1) of the system for the two
> versions of quantum espresso (6.5 and 6.4 : with same compilers) are:
>              (QE 6.5)           (QE 6.4)
>     1      -207.654081    -207.686563
>      2      -63.775769      -61.840758
>      3      -55.090067      -57.158594
>      4     -15.183543       -15.903109
>      5      20.311666        21.386665
>      6     129.683418       129.214185
>      7     615.352300        615.209434
>      8     676.597853        676.547298
>      9     999.285772        999.331118
>     10   1071.840671      1071.779138
>     11   1253.496419      1253.514267
>     12   1350.618940      1350.925730
>     13   1735.698557      1735.582574
>     14    2965.285270     2965.885980
>     15    3665.961548     3665.948232
>
> The maximum difference in any frequency for the two versions is about 2
> cm-1. My question:
>  Is the difference of 2 cm-1 within the error limit of ph.x calculations?
> What is the accuracy we expect in reporting the vibrational frequency (via
> ph.x)?
>
> Regards and Thanks
> Unmesh Mondal
> IISER Pune, India
>
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-- 
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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