Yambo Community Forum

Dear all:
After I have performed a GW calculation with ppa method, I want to include these results in my optical spectra calculations. I issue the command “yambo -o c -k alda -V qp” and generate the following input. However, the results are the same as “yambo -o c -k alda”. So what is the problem or how can I correct it? Thank you!

optics # [R OPT] Optics
chi # [R CHI] Dyson equation for Chi.
tddft # [R K] Use TDDFT kernel
Chimod= "ALDA" # [X] IP/Hartree/ALDA/LRC/BSfxc
FxcGRLc= 847 RL # [TDDFT] XC-kernel RL size
NGsBlkXd= 847 RL # [Xd] Response block size
% QpntsRXd
1 | 8 | # [Xd] Transferred momenta
%
% BndsRnXd
1 | 400 | # [Xd] Polarization function bands
%
% EnRngeXd
0.00000 | 10.00000 | eV # [Xd] Energy range
%
% DmRngeXd
0.10000 | 0.10000 | eV # [Xd] Damping range
%
ETStpsXd= 100 # [Xd] Total Energy steps
% LongDrXd
1.000000 | 0.000000 | 0.000000 | # [Xd] [cc] Electric Field
%
XfnQPdb= "E < ./ndb.QP" # [EXTQP Xd] Database
XfnQP_N= 1 # [EXTQP Xd] Interpolation neighbours
% XfnQP_E
0.000000 | 1.000000 | 1.000000 | # [EXTQP Xd] E parameters (c/v) eV|adim|adim
%
% XfnQP_Wv
0.00 | 1.00 | 0.00 | # [EXTQP Xd] W parameters (valence) eV|adim|eV^-1
%
% XfnQP_Wc
0.00 | 1.00 | 0.00 | # [EXTQP Xd] W parameters (conduction) eV|adim|eV^-1
%
XfnQP_Z= ( 1.000000 , 0.000000 ) # [EXTQP Xd] Z factor (c/v)


Shijun Zhao
Peking University

Dear Shijun,
beside the fact that an ALDA spectrum including QP energies does not make much sense to me,
in general please post your report and output in order to have a look what is going wrong.
In this case I so suspect that your ndb.QP database is found in SAVE directory (as by default) and the correct setting of the input variable should be: Best,

Daniele

Sorry, but could you give me a brief explanation taht why ALDA spectrum including QP energies does not make sense? I know BSE could be the correct way but my computer resources are not allowed me to perform such calculations.
I have placed the ndb.QP file in the same directory as yambo.in file. I have checked the log file and the ndb.QP has been included. The obtained spectra varied little but in reasonable agreement with the small correction from GW calculations.

Dear Shijun,
you can have a look on how correlation effects are included in Time dependent DFT, and Many Body perturbation theory, see e.g. Onida, Reining, Rubio, Rev. Mod. Phys. 74, 601 (2002) or other textbook. QP energies are charged excitations (energies needed to add/remove an electron from the system) and if you want to calcualte absorption you need to include quasi-electron quasi hole attraction (excitons, bethe salpeter) and ALDA does not contain such interaction. On other hand ALDA include exchange and correlation in the response of the KS system (so no QP energies are needed here), but it is well known that ALDA is unsatisfactory for bulk semiconductors. Finally please note that a Bethe Salpeter calculation should be not heavier of the calculation you are now doing in linear response.

Best,
Daniele