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[cc-lee@imre.a-star.edu.sg]

Dear all,

Currently, I am faced with some problem in optimizing a structure. The 
structure comprises 2 layer of Co on top of CoSi2(111) and on top of Co 
layer, there is a layer vacuum. The purpose of the calculation is to compare 
the structure of various model in term of the energy. For this model, I 
intend to relax the structure of the supercell with 2 layer of Co first and 
then I will add one more layer of Co to the relaxed structure at site A (hcp) 
and site C (cubic). Then I will relax the two structure again. But I have 
problem in using optcell =2. The program will always terminate when it begins 
BROYDEN STEP NUMBER 1. The new acell parameter for this step is very strange.

BROYDEN STEP NUMBER   1  
------------------------------------------------------
 Unit cell characteristics (before scfcv) :
  acell=  1.2996230033E+02  3.3830421286E+02  8.4974410439E+02
  rprim=  4.8279694872E-01 -8.4609172241E-01  2.2591127368E-01
          6.5002230740E-01  7.5493095490E-01 -8.6892193096E-02
         -9.0697866292E-02 -3.3826661261E-01  9.3666941652E-01

The structure seems to be distorted. Any suggestions? The input file for this 
calculation is almost the same as below except optcell=2 and iscf = 5. The 
program also suggested to increase the value of dilatmx. The suggested value 
is far too big to be considered reasonable.(You need at least dilatmx=    
1.062897E+02)

Then I try with optcell=3 and below is my input file. But the program also 
terminated at step 1 and suggested to increase the dilatmx value. 


-----------------------------------------------------------------------
optcell 3
ionmov 2
ntime 50
dilatmx 1.1 
ecutsm 1.50

#Definition of the unit cell
acell 2*7.5644 19.832230052577302  
rprim  sqrt(0.75) -0.5 0.0 1.0 0.0 0.0 0.0 0.0 1.0 

#Definition of the atom types
ntypat 2          
znucl 14  27      

#Definition of the atoms
natom 42               
typat 16*1 26*2             
xred                  
0.2500000000000000   0.0000000000000006   0.4078287387218850
0.2500000000000000   0.4999999999999999   0.4078287387218850
0.7500000000000001   0.0000000000000005   0.4078287387218850
0.7500000000000001   0.5000000000000000   0.4078287387218850
0.0833333333333333   0.1666666666666670   0.2521148648812809
0.0833333333333334   0.6666666666666670   0.2521148648812809
0.5833333333333330   0.1666666666666670   0.2521148648812810
0.5833333333333330   0.6666666666666670   0.2521148648812810
0.0833333333333333   0.1666666666666670   0.4856856756421870
0.0833333333333333   0.6666666666666669   0.4856856756421870
0.5833333333333330   0.1666666666666670   0.4856856756421870
0.5833333333333330   0.6666666666666670   0.4856856756421870
0.4166666666666671   0.3333333333333340   0.3299718018015829
0.4166666666666671   0.8333333333333340   0.3299718018015829
0.9166666666666671   0.3333333333333340   0.3299718018015830
0.9166666666666671   0.8333333333333340   0.3299718018015830
0.2500000000000000   0.0000000000000002   0.2910433333414320
0.2500000000000000   0.4999999999999999   0.2910433333414320
0.7500000000000001   0.0000000000000002   0.2910433333414320
0.7500000000000001   0.5000000000000000   0.2910433333414320
0.4166666666666671   0.3333333333333330   0.4467572071820359
0.4166666666666671   0.8333333333333329   0.4467572071820359
0.9166666666666671   0.3333333333333340   0.4467572071820359
0.9166666666666671   0.8333333333333330   0.4467572071820359
0.8333333424430621   1.0000000036735399   0.5865316215946980
0.5000000271420410   1.0000000492645300   0.5865316215946980
0.1666666940373520   1.0000000216868401   0.5865316215946980
0.8333333428062382   0.6666667094018280   0.5865316215946980
0.5000000271420410   0.6666667102026110   0.5865316215946980
0.1666666940373520   0.6666666797132340   0.5865316215946980
0.1666666940373520   0.3333333850173860   0.5865316215946980
0.5000000275204040   0.3333333728924820   0.5865316215946980
0.8333333428062382   0.3333333674282221   0.5865316215946980
0.9444444600329941   0.8888889232427570   0.6873775675472100
0.6111111083527250   0.8888888868709211   0.6873775675472099
0.2777777930668910   0.8888889317333120   0.6873775675472099
0.9444444600329941   0.5555555891487770   0.6873775675472100
0.6111111083527250   0.5555555921750720   0.6873775675472099
0.2777777930668910   0.5555555897597060   0.6873775675472099
0.2777777930668910   0.2222222556657260   0.6873775675472099
0.6111111083527250   0.2222222502014660   0.6873775675472099
0.9444444600329941   0.2222222550547971   0.6873775675472100

#Definition of the planewave basis set
ecut  35.0        
tsmear 0.001  
nband 500
iprcel 45

#Definition of the k-point grid
kptopt 0          
nkpt 1           
kpt 0.0 0.0 0.0

#Definition of the SCF procedure
iscf 3
nstep 200         
toldfe 1.0d-6     
tolmxf 1.0d-6
#Exchange-correlation functional
ixc 11

---------------------------------------------------------------------

The job is terminated with the following error.

chkdilatmx: ERROR -
  The new primitive vectors rprimd (an evolving quantity)
  are too large with respect to the old rprimd and the accompanying dilatmx :
  this large change of unit cell parameters is not allowed by the present 
value of dilatmx.
  You need at least dilatmx=    1.780394E+00
  Action : increase the input variable dilatmx.

 leave_new : decision taken to exit ...



BROYDEN STEP NUMBER   1  
------------------------------------------------------
 Unit cell characteristics (before scfcv) :
  acell=  9.2879094130E+00  9.9310758408E+00  1.1980650907E+01
  rprim= -9.5088242859E-01  2.9217878180E-01  1.0224561837E-01
         -5.4656217067E-01  8.3192737940E-01 -9.5742514068E-02
         -2.0807398153E-01 -7.7602704006E-01 -5.9538831976E-01
  ucvol=  5.6740156304E+02 Bohr^3


The new acell at step 1 seems to change a bit too much especially at 
acell(c). I don't really expect acell to change significantly. Any suggestion 
to improve or I can just increase dilatmx for this input file?

Can anyone describe to me what is the different between optcell=1 and 
optcell=3?

Cheers,
ChinChai