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["Ane Xu" <xzp02@mails.tsinghua.edu.cn>]

Dear all:
    I am conducting structure optimization on a unitcell of carbon
nanotube, using following parameters (ecut and k-points are optimized
for energy), but in my log file I found these problems.
1) 
/*
  getcut: wavevector=  0.0000  0.0000  0.0000  ngfft=  90  90  54
         ecut(hartree)=     35.000   => boxcut(ratio)=   2.41416

 getcut : COMMENT -
  Note that boxcut > 2.2 ; recall that boxcut=Gcut(box)/Gcut(sphere) = 2
  is sufficient for exact treatment of convolution.
  Such a large boxcut is a waste : you could raise ecut
  e.g. ecut=   50.996504 Hartrees makes boxcut=2
*/

  But 500eV is enough in some reference papers(using VASP, Castep...),
so I wonder how can I do to correct this "waste" ? Raise ecut (I do not
want the simulation to be much slower), reduce ngfft manually (anything
else to be considered?), or in some other manner?

2) 
/*
 fconv : at Broyd/MD step  24, gradients have not converged yet.
  max grad (force/stress) = 4.3791E-03 > tolmxf= 5.0000E-05 ha/bohr
(free atoms) 
*/
   The grad is too large after 24 steps! Is my tolmxf too large? maybe
result from 5.0D-3 or 5.0D-4 is convictive?

3)
/*
 Non-SCF iterations; k pt #    2  k=  0.50000  0.50000  0.37500  band
residuals:
 res:  4.72E-10  3.31E-10  4.48E-10  4.66E-10  3.22E-10  4.33E-10
3.62E-10  5.60E-10 
......
At SCF step    9, etot is converged :
  for the second time, diff in etot=  7.844E-08 < toldfe=  1.000E-06 
*/
    A simple question, why the iteration step is called "Non-SCF" and
"SCF" at the same time?

---------------
ndtset 1
prtgeo 1

optcell 0
ionmov 3

ntime  30
dilatmx 1.20
nstep  20
toldfe 1.0d-6
diemac 1.0d0
getwfk -1

kptopt 1
nshiftk 1
shiftk  0.5 0.5 0.5

ngkpt   1  1  4
#intxc 1
ecut  35

acell     7.4080000000    7.4080000000    4.2080000000 angstrom
angdeg   90.0000000000   90.0000000000   72.4600000000

ntypat 1
znucl  6                      
natom 20
typat 20*1
xangst
-1.933  -0.302  -1.952
-1.744   0.888  -1.242
-0.889   1.736  -1.952
-0.889   1.736   0.887
-1.384  -1.376  -1.242
-1.384  -1.376   0.177
-1.933  -0.302   0.887
-1.744   0.888   0.177
 1.744  -0.888  -1.952
 0.889  -1.736  -1.242
-0.306  -1.931  -1.952
-0.306  -1.931   0.887
 1.933   0.302  -1.242
 1.933   0.302   0.177
 1.744  -0.888   0.887
 0.889  -1.736   0.177
 0.306   1.931  -1.242
 1.384   1.376  -1.952
 1.384   1.376   0.887
 0.306   1.931   0.177 
---------------


best wishes

ZP

---------------------------------------------
Xu ZhiPing
Ph.D Candidate
Office: 3-405
Tel    : +86-10-627-83814
Department of Enginnering Mechanics
Tsinghua University
Beijing, CHINA, 100084
---------------------------------------------