# [Pw_forum] symmetries during a vc-relax--> Non zero values in celldm !!

JAY Antoine Antoine.JAY at isae-supaero.fr
Tue Oct 3 08:32:35 CEST 2017

If symmetries are conserved, why do I obtained non zero termsÂ  in the CELL_PARAMETERS where there should be zero terms?
Here is a full example in which i asked for relaxation of a monoclinic base-center system with cell parameters
v1 = ( a/2, 0, -c/2),
v2 = (b*cos(gamma), b*sin(gamma), 0),
v3 = ( a/2, 0, c/2)
but in the output v1(2) and v3(2) are not exactelly null!!
To obtain the celldm, if I use the reversed equations
celldm(2)=b/a
celldm(3)=c/a,
celldm(4)=cos(ab)
I do not take into account these non zero values, which then gives the wrong atomic positions.
Fortunatelly, the loose of symmetry is small, and if use these "wrong" celldm in a second vc-relax, then third and fourth,
until the relaxation becomes useless,Â  I finally obtain v1(2) and v3(2) exactelly null so the reversed equations gives the right celldm.
But this is not the good way to do it as one vc-relax that concerve the symmetries the should be the good way.

My input/output example:
as input:Â
&system
Â Â Â  ibrav = 13,
Â Â Â  celldm(1) = 16.288976472
Â Â Â  celldm(2) = 0.544519937
Â Â Â  celldm(3) = 0.668172534
Â Â Â  celldm(4) = 0.501142811
Â Â Â  nat=Â  13,
Â Â Â  ntyp= 2,Â Â Â
Â Â Â  ecutwfc =80,
Â Â Â  occupations='smearing',
Â Â Â  smearing='mp',
Â Â Â  degauss=0.0015,
/
Â &electrons
Â Â Â  diagonalization='david'
Â Â Â  mixing_beta = 0.7
Â Â Â  conv_thr =Â  4.0d-13
Â Â Â  electron_maxstep=800
/
&ions
Â Â Â  ion_dynamics='damp'
Â Â Â  pot_extrapolation='second_order'
Â Â Â  wfc_extrapolation='second_order'
Â /
&CELL
Â Â Â  cell_dynamics='bfgs',
Â Â Â  press = 0,
Â Â Â  press_conv_thr=0.1D0
Â Â Â  cell_factor=2
ATOMIC_SPECIES
BÂ  10.810 B-EB.pw
CÂ  12.011 C-EB.pw
ATOMIC_POSITIONS {crystal}
BÂ Â Â Â Â Â  -0.188793817Â  -0.218234974Â Â  0.307727082
BÂ Â Â Â Â Â  -0.192719364Â Â  0.340793252Â  -0.192719364
BÂ Â Â Â Â Â Â  0.307727082Â  -0.218234974Â  -0.188793817
BÂ Â Â Â Â Â Â  0.190169938Â Â  0.219331860Â  -0.303953219
BÂ Â Â Â Â Â Â  0.194974825Â  -0.339438271Â Â  0.194974825
BÂ Â Â Â Â Â  -0.303953219Â Â  0.219331860Â Â  0.190169938
BÂ Â Â Â Â Â Â  0.007880732Â  -0.005471582Â Â  0.338682645
BÂ Â Â Â Â Â Â  0.010098557Â Â  0.335888236Â Â  0.010098557
BÂ Â Â Â Â Â Â  0.338682645Â  -0.005471582Â Â  0.007880732
BÂ Â Â Â Â Â  -0.005624351Â Â  0.003265079Â  -0.333219342
CÂ Â Â Â Â Â  -0.003954045Â  -0.319654369Â  -0.003954045
BÂ Â Â Â Â Â  -0.333219342Â Â  0.003265079Â  -0.005624351
CÂ Â Â Â Â Â Â  0.425826764Â Â  0.426219771Â Â  0.425826764
K_POINTS {automatic}
12 12 12 1 1 1

as output:
CELL_PARAMETERS (alat= 16.28897647)
Â Â  0.498490550Â Â  0.013603023Â  -0.308425490
Â Â  0.284663797Â Â  0.492181426Â  -0.000000000
Â Â  0.498490550Â Â  0.013603023Â Â  0.308425490
ATOMIC_POSITIONS (crystal)
BÂ Â Â Â Â Â  -0.202019099Â  -0.212621756Â Â  0.346315439
BÂ Â Â Â Â Â  -0.205370945Â Â  0.354119566Â  -0.205370945
BÂ Â Â Â Â Â Â  0.346315439Â  -0.212621756Â  -0.202019099
BÂ Â Â Â Â Â Â  0.206012998Â Â  0.209180405Â  -0.314139284
BÂ Â Â Â Â Â Â  0.204538774Â  -0.320989968Â Â  0.204538774
BÂ Â Â Â Â Â  -0.314139284Â Â  0.209180405Â Â  0.206012998
BÂ Â Â Â Â Â Â  0.005743670Â Â  0.002927514Â Â  0.339636302
BÂ Â Â Â Â Â Â  0.004144354Â Â  0.351749983Â Â  0.004144354
BÂ Â Â Â Â Â Â  0.339636302Â Â  0.002927514Â Â  0.005743670
BÂ Â Â Â Â Â Â  0.006794941Â  -0.000361453Â  -0.329132341
CÂ Â Â Â Â Â Â  0.002164917Â  -0.321786041Â Â  0.002164917
BÂ Â Â Â Â Â  -0.329132341Â  -0.000361453Â Â  0.006794941
CÂ Â Â Â Â Â Â  0.382406681Â Â  0.380246423Â Â  0.382406681

Best regards,
Antoine Jay

On Monday, October 02, 2017 21:43 CEST, Paolo Giannozzi <p.giannozzi at gmail.com> wrote:
Â On Mon, Oct 2, 2017 at 4:25 PM, JAY Antoine <Antoine.JAY at isae-supaero.fr> wrote:Â I would like to perform a vc-relax that conserves the point group of each atom and the symmetry of the cell.Â vc-relax conserves the starting symmetry of the crystal. There is no way to conserve a symmetry that is not there at the beginning of the structural optimization.
Â For example,Â  in a trigonal cell, ibrav 5 imposes the symmetries:
v1 = a(tx,-ty,tz)
v2 = a(0,2ty,tz)
v3 = a(-tx,-ty,tz)
but these symmetries seem to be broken by the relaxation,Â see above: symmetries that were present in the starting structure are not broken. If a "symmetry" is "broken by structural relaxation", it wasn't a symmetry.
Â so that at the end of the relaxation, v2(1) is not exactly null and v1(1) is not exactly -v3(1),
or for the atomic positions, a coordinate (x, x, x) alat becomes (x, 0.99 x, 1.001x ).

?? Is it possible to impose that these constraints on the unit-cell and on the atomic positions stay fix during the relaxation process ??

This is a major issue for the use of these relaxed values in a second step for phonons for example, because it will then not detect the symmetries.
I think it could be a good idea to add into the vc-relax output file a lign that gives the corresponding celldm(i) and not only the CELL_PARAMETERS.Â I also think that it is a good idea, but it takes a bit of time and effort, that becomes a byte, or even a word (32 or 64 bits) if one wants to write a piece of code that works perfectly in all cases
Â PaoloIn fact, in the scf before the phonon use, we would like to have ibrav =/0 and celldm in state of ibrav 0 and quasi symmetric CELL_PARAMETERS...

Best regards,

Antoine Jay
Â

--Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222
Â

Â

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