[Pw_forum] WARNING! very low DOS at Fermi energy @ d3q code

[Lorenzo Paulatto]

On Monday, 17 April 2017 06:44:00 CEST Yusuf Zuntu wrote:
>  Dear All,
> Please help me resolve this problem "WARNING! very low DOS at Fermi energy".
> Below are my d3q calculation input and output. Thank you so much for
> anticipated quick response. Input d3q.....

Dear Yusuf,
the warning is exactly what it says: the density of states at the Fermi energy 
is very low. This usually indicates that the system you are studying is an 
insulator, but you're treating it as a metal. Or (less common) it may be a 
metal but you do not have any k-point in the proximity of the Fermi surface.

When doing a linear response calculation at Gamma, either phonon or d3, there 
are some terms that depend on the shift of the Fermi energy, this is finite in 
this case, but as the ratio of two divergent terms. In phonon it is usually 
multiplied by a factor that kills it, but in D3 there is a supposed a 
cancellation of different terms which may cause an important loss of precision.

If the system is actually insulator, you should treat it as such, i.e.e 
without smearing. If you have troubles converging it without smearing, you can 
converge with smearing first, than do an NSCF calculation without.

hth


-- 
Dr. Lorenzo Paulatto 
IdR @ IMPMC -- CNRS & Université Paris 6
phone: +33 (0)1 442 79822 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/423 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05