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

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

Best regards,
Antoine Jay





On Monday, October 02, 2017 21:43 CEST, Paolo Giannozzi <p.giannozzi at gmail.com> wrote:
 On Mon, Oct 2, 2017 at 4:25 PM, JAY Antoine <Antoine.JAY at isae-supaero.fr> wrote: I would like to perform a vc-relax that conserves the point group of each atom and the symmetry of the cell. 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.
 For example,  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, 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.
 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. 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
 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
 

--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
 


 

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