[QE-users] Time difference between PBE and PBE0 - PBE0 too long (not finishing) ...

[Giuseppe Mattioli]

Dear Michal
There are a few tricks to run EXX simulations in QE

First of all, in my experience best results are obtained by using  
norm-conserving pseudopotentials. I've not tested US or PAW  
implementations recently, but they used to be unstable and slower than  
NC, even if NC requires much higher ecutwfc.

Then

ecutwfc=90.0, (<-- this is the converged value for *my* pseudopotentials...)
ecutfock=90.0,

ecutfock is the cutoff for the Fock operator, it *strongly* affects  
the calculation time&resources. As a rule of thumb ecutfock=ecutwfc is  
a sensible choice for geometry optimizations and  
ecutfock=1.5~2*ecutwfc is a sensible convergence range for accurate  
electronic properties. The default value (ecutfock=ecutrho) is a huge  
loss of time and resources!

adaptive_thr=.true.

ensures a significant speedup for EXX loops.

I used this setup for the geometry optimization of a cyclic dipeptide  
(27 atoms, 78 electrons) on a 4-cores/16 GB RAM system with QE 6.6

  &control
     calculation = 'relax'
     restart_mode='from_scratch',
     prefix='$FILE',
     pseudo_dir = '$PSEUDO_DIR/',
     outdir='$TMP_DIR/',
     tprnfor=.true.,
     nstep=200,
     disk_io='low',
     wf_collect=.true.,
     max_seconds=40000,
  /
  &system
     ibrav=1, celldm(1)=40.0000,
     nat=27 ntyp=4,
     ecutwfc=90.0,
     ecutfock=90.0,
     occupations='smearing', degauss=0.01, <-- comment it out if you  
don't need unoccupied orbitals
     nspin=1,
     input_dft='b3lyp'
     vdw_corr='grimme-d3',
     assume_isolated='mp', <-- useful for an estimate of the vacuum level
  /
  &electrons
     diagonalization='david',
     mixing_mode='plain',
     mixing_beta=0.1,
     conv_thr=1.0d-6,
     electron_maxstep=100
     scf_must_converge=.false., <-- when convergence of the first GGA  
step is very difficult, use with care!
     adaptive_thr=.true.
  /
  &ions
     ion_dynamics='bfgs'
  /
ATOMIC_SPECIES
O    15.999     O.blyp-mt.UPF
N    14.007     N.blyp-mt.UPF
C    12.011     C.blyp-mt.UPF
H     1.008     H.blyp-vbc.UPF
ATOMIC_POSITIONS {angstrom}
...
K_POINTS {gamma}

The output should print the expected memory usage (it is often  
slightly overestimated)

      Estimated total dynamical RAM >       7.35 GB

As you can see, such calculation requires up to 8 GB RAM, and a little  
more if you increase ecutfock to 135 Ry (i.e., 1.5*ecutwfc). Keep in  
mind that QE is not designed for calculations of isolated systems  
surrounded by a lot of vacuum space, and it uses for small molecules  
many more resources than a GTO code. Nevertheless, I've satisfactorily  
performed QE simulations of a lot of small and large molecular systems  
using EXX functionals such as B3LYP or HSE...

HTH
Giuseppe

Quoting Husak Michal <Michal.Husak at vscht.cz>:

> I would like to ask what is the typical time difference between  
> running PBE and
>
> and the hybrid PBE0 ...
>
>
> Using CASTEP the PBE0 takes typically 200-1000 x more time than PBE ...
>
> This is OK , I run CASTEP on a 1000 node supercomputer and work is  
> done in a few
>
> days typically ...
>
>
> I had run PBE and PBE0  scf on super simple system (Al generated by  
> Material Cloud www, diamond generated by me) ...
>
> I run in  MP mode on 4 core CPU system ...
>
> PBE - 7.5 second
>
> PBE0 - run 24 hours and not yet finished
>
>
> PBE0 on the target molecule (alanine) crashed immediately with  
> segmentation failure ...
>
>
> Do I something wrong, is there some magic in setting hybrid  
> functional or do i relay need a supercomputer ? I believed I can  
> just add PBE0 request to tested PBE setup ...
>
> Can anybody send me a working sample PBE0 calculation setup for  
> small organic molecular crystal ?
>
>
> Michal Husak
>
> UCT Prague
> _______________________________________________
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GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
Via Salaria Km 29,300 - C.P. 10
I-00015 - Monterotondo Scalo (RM)
Mob (*preferred*) +39 373 7305625
Tel + 39 06 90672342 - Fax +39 06 90672316
E-mail: <giuseppe.mattioli at ism.cnr.it>