[Pw_forum] nstep, npool and FFT grid

[brsahu at physics.utexas.edu]

Dear pwscf users,

I have two questions regarding

1) FFT grid

In a system where the FFT-grids along x-, y- and z-directions are not
equal,which one of these decides the PW parallelization efficiency?

I have a system with about 200 atoms. In the output, I get

G cutoff =  164.2214  (2183159 G-vectors)     FFT grid: ( 27,***,144)
G cutoff =   43.7924  ( 301345 G-vectors)  smooth grid: ( 15,625, 72)

There are '***' printed for the y-dir FFT grid.

This latest version of the pwscf code is compiled on a linux cluster  
(cvs version).

Can something be changed (array declaration for FFT) so that it can
print FFT grid for large systems so that I can tune the number of  
processors for
G-vector and/or k-pt parallelization.

2) nstep and npool

nstep (INPUT_PW file in Doc) says is the number of ionic+electronic  
steps which is
1 for scf, nscf, and bands  and 0 for neb etc. and 50 for "relax"  
vc-relax etc by default.

I have a run with "relax". I put nstep=250 does it mean it will do 250  
ionic steps
? Is there a default for how about the electronic steps? I could not  
find a separate tag
for electronic steps.

Also How do one estimate whether k-point parallelization will be  
faster or G-vector(FFT)
grid parallelization is faster for a given system. The suggestion for  
parallelization
issues in the usersguide suggest that choose number of processors such  
that the third FFT
grid is divisible by number of processors chosen. and choose number of  
pools of
processors so that number of k-points are divisible by that. Is there  
a default for
number of pools of processors that a running job assumes if not  
specified  in the "job"
submitting script or one has to specify it explicitly?

Is number of pools of processors divides the number of processors  
specified in the "job"
submitting and # of proc./# of pools = the number of processors that  
should be divisor of
the third FFT grid?



Bhagawan Sahu
Microelectronics Research Center
University of Texas at Austin
10100 Burnet Road, Bldg. 160
MER 1.606H, Mail code R9900, TX 78758-4445
e-mail: brsahu at physics.utexas.edu
Phone: (512)-471-1344 FAX: (512)-471-8575