No subject

<br>
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September/010123.html</a><br>
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<br>
I understand that the third derivatives were never implemented and<br>
that only the LDA part was calculated, for the Perdew Zunger<br>
parameterization. Is this third derivative the third derivative that<br>
is mentioned in the routine PH/raman_mat.f90 (i.e, the third<br>
derivative of the energy with respect to the atomic displacements and<br>
with respect to two electric fields, as described in Lazzeri and<br>
Mauri, PRL 90,036401)?<br>
<br>
I am interested in getting a feel for what would be needed to<br>
implement the third derivatives with GGA into the code, as well as<br>
what sort of errors one might expect when using the third-order term<br>
as currently coded with LDA pseudopotentials (comments in the forums<br>
have hinted that the errors are likely small). =A0The shift in the Raman<br=
>
spectra for cd-Si is pretty minor when using a GGA pseudo compared to<br>
that of an LDA calculation, but I wanted to be able to quantify the<br>
error better for other systems where comparing the GGA raman spectra<br>
to one calculated within LDA was not possible (e.g., when the system<br>
doesn&#39;t exhibit a gap in LDA).<br>
<br>
Thanks for the help!<br>
<br>
Best,<br>
Brad<br>
UC Berkeley<br>
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