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

Paolo Giannozzi p.giannozzi at gmail.com
Tue Oct 3 11:37:00 CEST 2017

```These are the only symmetries that are detected in your input, and the only
ones that are conserved during the relaxation:

2 Sym. Ops. (no inversion) found

s                        frac. trans.

isym =  1     identity

cryst.   s( 1) = (     1          0          0      )
(     0          1          0      )
(     0          0          1      )

cart.    s( 1) = (  1.0000000  0.0000000  0.0000000 )
(  0.0000000  1.0000000  0.0000000 )
(  0.0000000  0.0000000  1.0000000 )

isym =  2     inv. 180 deg rotation - cart. axis [0,0,1]

cryst.   s( 2) = (     0          0          1      )
(     0          1          0      )
(     1          0          0      )

cart.    s( 2) = (  1.0000000  0.0000000  0.0000000 )
(  0.0000000  1.0000000  0.0000000 )
(  0.0000000  0.0000000 -1.0000000 )

point group C_s (m)
there are  2 classes
the character table:

E     s
A'     1.00  1.00
A''    1.00 -1.00

the symmetry operations in each class and the name of the first
element:

E        1
identity
s        2
inv. 180 deg rotation - cart. axis [0,0,1]

Paolo

On Tue, Oct 3, 2017 at 8:32 AM, JAY Antoine <Antoine.JAY at isae-supaero.fr>
wrote:

> 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
>
> Paolo
>
>> In 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 <+39%200432%20558216>, fax +39-0432-558222
> <+39%200432%20558222>
>
>
>
>
>
>
>
>
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--
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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