[Pw_forum] vc-relax - negative and non-zero cell parameters

[Максим Попов]

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>

> 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
> ===========================================================
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>



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