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.mon...@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 > > _______________________________________________ > Quantum ESPRESSO is supported by MaX (www.max-centre.eu/quantum-espresso) > users mailing list users@lists.quantum-espresso.org > https://lists.quantum-espresso.org/mailman/listinfo/users -- 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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