[QE-users] Negative dr2 error in DFT+U

[Dominik Gresch]

Dear QE community,

 

I am trying to run DFT+U calculations (lda_plus_u_kind=0) with a negative Ueff = U - J. However, this frequently results in the following error being raised:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     Error in routine mix_rho (1):
     negative dr2
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

I have traced this to the ns_ddot function in PW/src/scf_mod.f90, which calculates a contribution to dr2 from the Hubbard term. Due to the negative U, this value also becomes negative. Consequently, the error can be avoided by changing Hubbard_U(nt) to ABS(Hubbard_U(nt)) in this function (in two places).

My question is this:

- Does this change make sense, or is the sign of ns_ddot relevant?

- Does it make sense physically to use negative U? These are empirical values, to match a known band structure.

Thank you and best regards,

Dominik Gresch

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