[Pw_forum] ?spam? Re: vc-relax - negative and non-zero cell parameters

[Christopher Heard]

Dear Maxim, 

The structure is chosen at random, as I simply wish to test the vc-relax procedure, so I don't expect it to have a particular symmetry, although in general, I am dealing with clusters of low symmetry anyway.

My problem is that it makes no sense to me that off diagonal elements can be non-zero, when they are relative. How can one have a value 0.000341145 x 0 (in the case of element (1,2))?  This, I do not understand.

Furthermore, what would a negative relative parameter mean?
I suspect I am misinterpreting the meaning of these alat cell parameter matrices, but I'm not sure how.


Chris


________________________________________
From: pw_forum-bounces at pwscf.org [pw_forum-bounces at pwscf.org] On Behalf Of Максим Попов [max.n.popov at gmail.com]
Sent: 20 May 2011 13:25
To: PWSCF Forum
Subject: ?spam? Re: [Pw_forum] vc-relax - negative and non-zero cell parameters

Dear Christopher,

1) the structure, in the input you provided, looks weird to me. Are you sure it is really what you intend to simulate?
2) The vc-relaxation process with symmetry conservation is driven by crystal symmetry. That is to say: cubic unit
cell does not necessary mean cubic symmetry. Coming back to your structure, by visual inspection one can see
that it does not possess cubic symmetry (it is P1, most probably). Therefore off-diagonal terms of Bravais matrix
evolved during cell-shape relaxation.

Best regards,
Maxim.

2011/5/20 Christopher Heard <CJH085 at bham.ac.uk<mailto:CJH085 at bham.ac.uk>>
Hi,
I have a question regarding vc-relaxation in relative coordinates.
If I input a cubic lattice and allow the program to relax the cell parameters, and print as relative coordinates of the original cell parameters (alat), I dont understand how off diagonal terms can be non-zero (what is 0 x 0.9 if not 0?) or how negative values have physical meaning.

Perhaps I am not understanding the method by which the calculation is made, but I couldn't get any understanding from the INPUT_PW.txt file or the manual, so I am asking the list.

Cheers,
Chris


========================================================
INPUT FILE

&CONTROL
 calculation  = "vc-relax",
 prefix       = "QETEST",
 pseudo_dir   = " /home/heard/pseudo ",
 outdir       = ".",
 nstep = 10,
/
&SYSTEM
 ibrav     = 0,
 nat       = 3,
 ntyp      = 2,
 ecutwfc   = 20.D0,
 occupations = 'smearing'
 smearing = 'mp'
 degauss = 0.06
/
&ELECTRONS
 electron_maxstep =  1000,
 conv_thr    = 1.D-6,
 mixing_beta = 0.3D0,
/
&IONS
/
&CELL
 cell_dynamics = 'bfgs',
/
CELL_PARAMETERS cubic
30.23  0.0  0.0
 0.0 30.23  0.0
 0.0  0.0 30.23
ATOMIC_SPECIES
Cu 63.5 Cu.pbe-d-rrkjus.upf
Ag 107.9 Ag.pbe-d-rrkjus.upf

ATOMIC_POSITIONS {crystal}

Cu  0.9110342051985  0.5412603911172  0.9652262133233
Cu  0.3046428810907  0.3592799679414  0.0488897975312
Ag  0.868139803195   0.7597487846068  0.8302658063157
K_POINTS {Gamma}


======================================================
START OF OUTPUT FILE

  bravais-lattice index     =            0
    lattice parameter (a_0)   =      30.2300  a.u.
    unit-cell volume          =   27625.7732 (a.u.)^3
    number of atoms/cell      =            3
    number of atomic types    =            2
    number of electrons       =        33.00
    number of Kohn-Sham states=           21
    kinetic-energy cutoff     =      20.0000  Ry
    charge density cutoff     =      80.0000  Ry
    convergence threshold     =      1.0E-06
    mixing beta               =       0.3000
    number of iterations used =            8  plain     mixing
    Exchange-correlation      =  SLA  PW   PBE  PBE (1434)
    EXX-fraction              =        0.00
    nstep                     =           10


    celldm(1)=  30.230000  celldm(2)=   0.000000  celldm(3)=   0.000000
    celldm(4)=   0.000000  celldm(5)=   0.000000  celldm(6)=   0.000000

    crystal axes: (cart. coord. in units of a_0)
              a(1) = (   1.000000   0.000000   0.000000 )
              a(2) = (   0.000000   1.000000   0.000000 )
              a(3) = (   0.000000   0.000000   1.000000 )

===========================================================
OUTPUT COORDINATES

    new unit-cell volume =  27844.52160 a.u.^3 (  4126.13235 Ang^3 )

CELL_PARAMETERS (alat= 30.23000000)
  1.003485881   0.000341145  -0.000166335
  0.000341140   1.002476151   0.000847099
 -0.000166325   0.000846982   1.001936910

ATOMIC_POSITIONS (crystal)
Cu       0.911117504   0.540084682   0.965438693
Cu       0.305212112   0.357448827   0.044305096
Ag       0.867487274   0.762755634   0.834638028
===========================================================
_______________________________________________
Pw_forum mailing list
Pw_forum at pwscf.org<mailto:Pw_forum at pwscf.org>
http://www.democritos.it/mailman/listinfo/pw_forum



--
Best regards, Max Popov
Ph.D. student
Materials center Leoben (MCL), Leoben, Austria.