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
