[Pw_forum] Na phonon issues
Paolo Giannozzi
giannozz at nest.sns.it
Wed May 23 17:43:11 CEST 2007
On May 17, 2007, at 16:30 , Miguel Martínez Canales wrote:
> I've been doing some phonon calculations on Na lately, and I've got
> an issue with the Na.pw91-n-mt.UPF pseudopotential [...]
> omega( 1) = -0.102259 [THz] = -3.411026 [cm-1]
> omega( 2) = 0.766410 [THz] = 25.564857 [cm-1]
> omega( 3) = 1.171147 [THz] = 39.065511 [cm-1]
> [...]
> If I use the LDA pseudo Na.pz-n-vbc.UPF near equilibrium
> (a=7.738a_0) with ecut=30 Ry and 18 18 18 shifted M-P grid, I get
> the following omegas:
> omega( 1) = 0.273215 [THz] = 9.113532 [cm-1]
> omega( 2) = 0.835358 [THz] = 27.864728 [cm-1]
> omega( 3) = 1.264994 [THz] = 42.195940 [cm-1]
>
> While the Na.pbe-sp-van_ak.UPF pseudo, with the same grid as the
> enclosed input (20 20 20 shifted M-P), ecut=35 and ecutrho=300 Ry,
> I get also sensible results (with 7.932 a_0 as lattice parametre):
> omega( 1) = 0.390200 [THz] = 13.015753 [cm-1]
> omega( 2) = 0.857166 [THz] = 28.592182 [cm-1]
> omega( 3) = 1.238921 [THz] = 41.326237 [cm-1]
>
> So I have the following question: Is the mechanical instability I
> see a product of my input or a product of the pseudopotential?
it might be the usual problem of the loss of translational invariance
at q=0, whose influence extends to small q as well; or it might be
related to the rather nasty numerical behaviour of the PW91 functional.
I would try the pw91 pseudopotential with lda (the pp file must be
edited),
then the lda pp with pw91 functional, just to see what happens...
Paolo
---
Paolo Giannozzi, Democritos and University of Udine, Italy
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