[Pw_forum] Details on "third order derivatives not implemented with GGA" error

[Brad Malone]

Hi, I am seeking more information on the error

> from phq_setup : error # 1
> third order derivatives not implemented with GGA

>From the forums:

http://www.democritos.it/pipermail/pw_forum/2008-September/010123.html
http://www.democritos.it/pipermail/pw_forum/2010-February.txt

I understand that the third derivatives were never implemented and
that only the LDA part was calculated, for the Perdew Zunger
parameterization. Is this third derivative the third derivative that
is mentioned in the routine PH/raman_mat.f90 (i.e, the third
derivative of the energy with respect to the atomic displacements and
with respect to two electric fields, as described in Lazzeri and
Mauri, PRL 90,036401)?

I am interested in getting a feel for what would be needed to
implement the third derivatives with GGA into the code, as well as
what sort of errors one might expect when using the third-order term
as currently coded with LDA pseudopotentials (comments in the forums
have hinted that the errors are likely small).  The shift in the Raman
spectra for cd-Si is pretty minor when using a GGA pseudo compared to
that of an LDA calculation, but I wanted to be able to quantify the
error better for other systems where comparing the GGA raman spectra
to one calculated within LDA was not possible (e.g., when the system
doesn't exhibit a gap in LDA).

Thanks for the help!

Best,
Brad
UC Berkeley