[Pw_forum] ?spam? Re: vcrelax  negative and nonzero 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 vcrelax 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 nonzero, 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_forumbounces at pwscf.org [pw_forumbounces 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] vcrelax  negative and nonzero 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 vcrelaxation 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 offdiagonal terms of Bravais matrix
> evolved during cellshape 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 vcrelaxation 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 nonzero (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 = "vcrelax",
> 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.D6,
> 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.pbedrrkjus.upf
> Ag 107.9 Ag.pbedrrkjus.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
>
> bravaislattice index = 0
> lattice parameter (a_0) = 30.2300 a.u.
> unitcell volume = 27625.7732 (a.u.)^3
> number of atoms/cell = 3
> number of atomic types = 2
> number of electrons = 33.00
> number of KohnSham states= 21
> kineticenergy cutoff = 20.0000 Ry
> charge density cutoff = 80.0000 Ry
> convergence threshold = 1.0E06
> mixing beta = 0.3000
> number of iterations used = 8 plain mixing
> Exchangecorrelation = SLA PW PBE PBE (1434)
> EXXfraction = 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 unitcell 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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