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

Stefano de Gironcoli degironc at sissa.it
Fri May 20 23:28:42 CEST 2011

```Dear Christopher

On 05/20/2011 03:02 PM, Christopher Heard wrote:
> 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.
>
I don't understand what you don't understand...
cell parameters are just the three fundamental lattice vectors defining
the Bravais lattice.
why should it be a problem to  have a vector with a negative component ?

stefano
-
Stefano de Gironcoli, SISSA and DEMOCRITOS

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