[Pw_forum] optimize monoclinic cell

[Axel Kohlmeyer]

On Wed, 2009-04-08 at 19:01 +0800, 程迎春 wrote:
> Dear Pwscf users,
>         I did a variable cell calculations for a base centered
> monoclinic crystal.  The cell displayed in xcrysden is correct, and
> PWscf can find the symmetry.  However, in the optimizing process, the
> cell parameter changed unexpectedly, which may be wrong.  In the

no. it is most likely not wrong, but rather an indication of the 
level of numerical accuracy with which you computed your forces 
and stress tensor. 

cheers,
   axel 

>  user's guide of PWscf, the base centered monoclinic cell is defined
> as follows:
> base centered monoclinic
> =============================
>    v1 = (  a/2,         0,                -c/2),
>    v2 = (b*cos(gamma), b*sin(gamma), 0),
>    v3 = (  a/2,         0,                  c/2),
> where gamma is the angle between axis a and b
> =============================
> In the first step of optimize process, the cell is changed as
> following:
> CELL_PARAMETERS (alat)
>    0.483414893   0.003687739  -0.271981740
>   -1.013180906   0.556415406   0.000000000
>    0.483414893   0.003687739   0.271981740
> My question is : why the quantities of second row of v1, v3 vector do
> not retain zero!
>  
> Any suggestion and comment is appriciated. Thank you in advance!
>  
> The input file is attached!
>  &CONTROL
>                        title = 'c2',
>                       prefix = 'li',
>                  calculation = 'vc-relax' ,
>                 restart_mode = 'from_scratch' ,
>                       outdir = './' ,
>                       wfcdir = './' ,
>                   pseudo_dir = './' ,
>                      tstress = .true. ,
>                      tprnfor = .true. ,
>                etot_conv_thr = 1.0D-5,
>                forc_conv_thr = 1.0D-4,
>  /
>  &SYSTEM
>                        ibrav = 13,
>                    celldm(1) = 14.7756,
>                    celldm(2) = 1.20528,
>                    celldm(3) = 0.56348,
>                    celldm(4) = -0.873177,
>                          nat = 12,
>                         ntyp = 1,
>                      ecutwfc = 200,
>                        nosym = .false. ,
>  /
>  &ELECTRONS
>             electron_maxstep = 100,
>                     conv_thr = 1.D-10,
>  /
>  &IONS
>  ion_dynamics='bfgs',
> /
>  &CELL
>  cell_dynamics='bfgs',
>  press=900,
> /
> ATOMIC_SPECIES
>    Li   6.914  Li.pz-s-mt.UPF
> ATOMIC_POSITIONS (crystal)
> Li   -0.46080   0.21740   1.49640
> Li   -0.02770   0.02590   1.28350
> Li   -0.80760   0.63690   0.91840
> Li    0.23010   0.43900   0.29270
> Li    0.06870   0.87600   0.40410
> Li   -0.04760   0.70260   1.56000
> Li   -1.49640  -0.21740   0.46080
> Li   -1.28350  -0.02590   0.02770
> Li   -0.91840  -0.63690   0.80760
> Li   -0.29270  -0.43900  -0.23010
> Li   -0.40410  -0.87600  -0.06870
> Li   -1.56000  -0.70260   0.04760
> K_POINTS automatic
>   8 8 8   1 1 1
> 
> -- 
> Y. C. Cheng
> Department of Phyics
> Nanjing University
> Nanjing 210093
> P. R. China
> Tel: 86-25-83592907
> Email: yccheng.nju at gmail.com
> 
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-- 
=======================================================================
Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.