[Pw_forum] The cell blowing up in MD simulation

[Tian Lan]

Thank you.

I did not notice the unit difference in pw.x and cp.x.

But it seems that the energies that both codes obtained agree well. I
pasted the results in the following. The first one is the result from
vc-relax operated on one unit cell, the total energy is -358.1 Ry, this is
very close to my original setup with scf calculation. The second one is
from cp.x operated on 2 by 2 by 2 supercell, with fixed atom positions(ion
and cell_dynamics are set to 'none'), the energy is -1410 au==-2820 Ry. I
don't know whether this is a big difference, but I cannot find an obvious
error, but the pressure is quite high, like100GPa, and it is sensitive to
the setup especially $electron part.

I don't quite get your point about the default positions of atoms in pw and
cp. In both codes, I used 'crystal', and in the supercell, all the ratios
are in reference to this cubic supercell. I don't think the atom positions
have much problem from the result.

Thank you!
Tian

---------------------------------------------------------------------------------------------
pw.x
------------------------------------------------------------------------------------------
total energy              =    -358.09688074 Ry
     Harris-Foulkes estimate   =    -358.09688074 Ry
     estimated scf accuracy    <          9.1E-14 Ry
     The total energy is the sum of the following terms:
     one-electron contribution =     -98.88874403 Ry
     hartree contribution      =      94.24811082 Ry
     xc contribution           =     -60.23256770 Ry
     ewald contribution        =    -293.22367983 Ry
     convergence has been achieved in  27 iterations
     Forces acting on atoms (Ry/au):
     atom   1 type  2   force =     0.00000000    0.00000000    0.00000000
     atom   2 type  2   force =     0.00000000    0.00000000   -0.00000000
     atom   3 type  2   force =     0.00000000   -0.00000000   -0.00000000
     atom   4 type  2   force =    -0.00000000    0.00000000   -0.00000000
     atom   5 type  1   force =     0.00000000    0.00000000    0.00000000
     atom   6 type  1   force =     0.00000000    0.00000000    0.00000000
     Total force =     0.000000     Total SCF correction =     0.000000

     entering subroutine stress ...
          total   stress  (Ry/bohr**3)                   (kbar)     P=
-0.08
  -0.00000054   0.00000000   0.00000000         -0.08      0.00      0.00
   0.00000000  -0.00000054   0.00000000          0.00     -0.08      0.00
   0.00000000   0.00000000  -0.00000054          0.00      0.00     -0.08
     bfgs converged in   5 scf cycles and   2 bfgs steps
     (criteria: energy < 0.10E-03, force < 0.10E-02, cell < 0.50E+00)

     End of BFGS Geometry Optimization

     Final enthalpy =    -358.0968807430 Ry
Begin final coordinates
     new unit-cell volume =    774.49676 a.u.^3 (   114.76858 Ang^3 )

CELL_PARAMETERS (alat=  9.15690000)
   1.002900997   0.000000000   0.000000000
   0.000000000   1.002900997   0.000000000
   0.000000000   0.000000000   1.002900997
ATOMIC_POSITIONS (crystal)
Ag       0.250000000   0.250000000   0.250000000
Ag       0.750000000   0.750000000   0.250000000
Ag       0.250000000   0.750000000   0.750000000
Ag       0.750000000   0.250000000   0.750000000
O        0.000000000   0.000000000   0.000000000
O        0.500000000   0.500000000   0.500000000
End final coordinates

------------------------------------------------------------------------------------
cp.x
------------------------------------------------------------------------------------
Partial temperatures (for each ionic specie)
   Species  Temp (K)   Mean Square Displacement (a.u.)
        1       0.00     0.0000
        2       0.00     0.0000
  nfi    ekinc  temph  tempp        etot      enthal       econs
econt    vnhh   xnhh0    vnhp   xnhp0
  100  0.00092    0.0    0.0 -1431.80272 -1431.80272 -1431.80272
-1431.80180  0.0000  0.0000  0.0000  0.0000
   writing restart file: ./Ag2O_51.save
   restart file written in   39.227 sec.

   Averaged Physical Quantities
                      accomulated      this run
   ekinc         :        7.61699       7.61699 (AU)
   ekin          :      374.63650     374.63650 (AU)
   epot          :    -1373.54937   -1373.54937 (AU)
   total energy  :    -1410.67335   -1410.67335 (AU)
   temperature   :        0.00000       0.00000 (K )
   enthalpy      :    -1410.67335   -1410.67335 (AU)
   econs         :    -1410.67335   -1410.67335 (AU)
   pressure      :       93.03124      93.03124 (Gpa)
   volume        :     6148.60237    6148.60237 (AU)

On Thu, Sep 13, 2012 at 1:22 AM, Paolo Giannozzi <giannozz at democritos.it>wrote:

> On Wed, 2012-09-12 at 15:33 -0700, Tian Lan wrote:
>
> > I expected a good structure from scf calculation should more or less
> > agree with the MD run.
>
> it does. First of all, you should verify that you can get the same
> energy (within numerical noise, and not forgetting the Ha to Ry
> conversion) with pw.x and cp.x (beware the different defaults
> for atomic positions in input). In the latter case, just perform
> an electronic minimization at fixed atoms.
>
> P.
> --
> Paolo Giannozzi, IOM-Democritos and University of Udine, Italy
>
>
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>



-- 
Lan, Tian
Ph.D. Candidate, Department of Applied Physics and Materials Science
California Institute of Technology,
Caltech M/C 138-78, Pasadena, CA, 91125
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