[Pw_forum] on variable cell optimization

Serge Nakhmanson nakhmans at physics.rutgers.edu
Fri Jan 28 16:58:54 CET 2005


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I have the same problem running version 2.1.2. I initially performed
a bfgs relaxation of the system with fixed volume and then started
volume relaxation reading in the "relaxed at fixed volume" configuration
as my starting point (was that a good idea?). This is what I have
at step 1 of vc-relax:

****************************************************************************

     Total force =     0.000555     Total SCF correction =     0.000032


     entering subroutine stress ...

          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.79
  -0.00000340  -0.00000002  -0.00000008         -0.50      0.00     -0.01
  -0.00000002   0.00020133   0.00000013          0.00     29.62      0.02
  -0.00000008   0.00000013  -0.00001863         -0.01      0.02     -2.74


     Parrinello-Rahman Damped Cell-Dynamics Minimization
     convergence thresholds: EPSE = 0.10E-03  EPSF = 0.10E-03  EPSP = 0.50E+00


     Entering Dynamics;  it =     1   time =  0.00000 pico-seconds


  new lattice vectors (alat unit) :
   0.999999998   0.000000000   0.000000000
   0.000000000   1.194324076   0.000000000
   0.000000000   0.000000000   1.144971993
  new unit-cell volume =   5824.5998 (a.u.)^3

*****************************************************************************

And this is my situation after 28 steps:

*****************************************************************************


     Total force =     0.000580     Total SCF correction =     0.000215


     entering subroutine stress ...

          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.64
  -0.00000407  -0.00000002  -0.00000009         -0.60      0.00     -0.01
  -0.00000002   0.00019975   0.00000012          0.00     29.38      0.02
  -0.00000009   0.00000012  -0.00001940         -0.01      0.02     -2.85


     Entering Dynamics;  it =    28   time =  0.00131 pico-seconds

  new lattice vectors (alat unit) :
   0.999999081  -0.000000004  -0.000000022
  -0.000000005   1.194369135   0.000000029
  -0.000000025   0.000000028   1.144967629
  new unit-cell volume =   5824.7920 (a.u.)^3

*****************************************************************************

This is the grep on pressure:

          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.79
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.78
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.80
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.78
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.77
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.81
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.79
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.81
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.80
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.73
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.79
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.76
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.79
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.77
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.77
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.72
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.78
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.77
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.79
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.77
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.77
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.78
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.77
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.76
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.66
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.77
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.75
          total   stress  (ryd/bohr**3)                  (kbar)     P=    8.64

Nothing is moving, nothing is changing. I expected at least some cell relaxation along
the y-axis. Do I actually need to kick the system to see some relaxation? Or is
my cell too heavy (see below)? I've seen people asking questions about a good value
for cell mass but no answers have been given. Is that something that can be looked up
in Allen-Tildesley? BTW, what is the UMA unit ( cell mass =  36.00000 UMA )?

My input file is given below.

THX,

Serge

  &control
     calculation = 'vc-relax'
     restart_mode='restart',
     prefix='PVDF96.relax',
     pseudo_dir = './',
     outdir = './',
     nstep = 100,
     tstress = .true.,
     max_seconds = 56000
     etot_conv_thr = 1.0d-4
     forc_conv_thr = 1.0d-4
  /
  &system
     ibrav=  8,
     nat=  96,
     ntyp= 3,
     celldm(1) =  16.210000,
     celldm(2) =   1.194324,
     celldm(3) =   1.144972,
     ecutwfc = 30.0,
     ecutrho = 270.0
     nosym=.true.
  /
  &electrons
     startingpot = 'file' ,
     startingwfc = 'file' ,
     mixing_mode = 'plain'
     mixing_beta = 0.3
     conv_thr =  1.0d-8
     diagonalization = 'david' ,
  &ions
     upscale=10
     ion_dynamics='damp'
     potential_extrapolation='wfc2'
  /
  &cell
     cell_dynamics='damp-pr'
     wmass = 36.00
  /
ATOMIC_SPECIES
  F  12.01 F_pbe_us.van.UPF
  C  12.01 C.pbe-van_bm.UPF
  H  12.01 H.pbe-van_bm.UPF
ATOMIC_POSITIONS {bohr}

[ ... cut ... ]

K_POINTS {gamma}


Paolo Giannozzi wrote:
> On Thursday 27 January 2005 23:25, Eduardo Ariel Menendez P wrote:
> 
>>I am trying to perorm a variable cell optimization
> 
> 
> version of the code?
> 

- --
******************************************************************************
 Serge M. Nakhmanson                                 phone: +1 (732) 445-4603
 Postdoctoral RA                                       fax: +1 (732) 445-4400
 Department of Physics & Astronomy
 Rutgers, The State University of New Jersey
 136 Frelinghuysen Road                          nakhmans at physics.rutgers.edu
 Piscataway, NJ 08854-8019          http://www.physics.rutgers.edu/~nakhmans/

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