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

[Offermans Willem]

Dear QE friends and Paolo,

I’m sorry to break into this mail thread, but could you elaborate a bit on “applying acoustic sum rule”.

How is this done?



Met vriendelijke groeten,
Mit freundlichen Grüßen,
With kind regards,


Willem Offermans
Researcher Electrocatalysis SCT
VITO NV | Boeretang 200 | 2400 Mol
Phone:+32(0)14335263 Mobile:+32(0)492182073

Willem.Offermans at Vito.be<mailto:Willem.Offermans at Vito.be>

[cid:982BA063-B96A-4A1B-89AB-5A01CA9FC70D at vito.local]

On 14 Sep 2020, at 08:34, Paolo Giannozzi <p.giannozzi at gmail.com<mailto:p.giannozzi at gmail.com>> wrote:

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<mailto: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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