[Pw_forum] Problems with EXX calculation and q-points

[Francesco Macheda]

Dear all,
I'm new to Quantum Espresso. I'm trying to get the energy bands from
Silicon bulk with hybrid functionals. I've read in previous posts and in
quantum documentation that "Their value (of the q-points) must be
compatible with the k-points used (that is k+q must be equivalent to some
other k in the k-points list)". I'm not certain to understand why it has to
be done this way. Moreover, and this is the main problem, I don't know how
to make them compatible. Once I have chosen a certain dimension of the
Monkhorst-Pack grid (nk1=nk2=nk3=K) which values can I give to
nqx1=nqx2=nqx3 ? Is there an a-priori argument?

Another error message that I get is "Error in routine electrons (1):
     dexx is negative!   Check that exxdiv_treatment is appropriate for the
system" in the calculation with B3LYP, while HSE seems working well up to a
certain cutoff, after which the error appears . I read that somebody solved
it changing exxdiv_treatment, but I don't if or why I should do that, or if
it is related to my "manual" choice of q-grid. Any suggestion?

Thanks for attention,
Francesco Macheda
Student at Pisa University
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