[Pw_forum] epsilon.f90 skips transitions involving similarly occupied states

Andrea Ferretti andrea.ferretti at unimore.it
Fri May 13 11:48:45 CEST 2016

Dear Roberto,

please note that some bug fixes and other minor changes have been 
recently (past few months) included in espilon. I would then 
recommend to refer to espresso-5.4.0 for both discussions and actual runs

take care

> I was reading through the epsilon.f90 code and I noticed that (5.2.1 version) at
> line 420 we have the following condition
> IF (abs(focc(iband2,ik)-focc(iband1,ik))< 1e-3) CYCLE
> this basically means that the contribution  to the dielectric function due to the transitions between
> iband2 and iband1 is neglected if the two states have nearly the same occupation. I removed this
> condition and noticed that, in some test case, states close to the Fermi level and closely spaced in energy would otherwise
> contribute a huge amount. This seems to be due to the fact that the denominator of eq. 10 ( epsilon.x manual ) becomes close
> to zero.
> I would like to ask experts of pwscf and epsilon.x, in particular, if the CYCLE condition was introduced for any specific
> physical or numerical reason and why it is safe to proceed this way.
> Thank you very much for your attention and have a nice day.
> Roberto Gaspari
> Ph.D.
> Compunet-Istituto Italiano di Tecnologia

Andrea Ferretti, PhD
S3 Center, Istituto Nanoscienze, CNR
via Campi 213/A, 41125, Modena, Italy
Tel: +39 059 2055322;  Skype: andrea_ferretti
URL: http://www.nano.cnr.it

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