[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>
>
>
>
>
>
>
>
>
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://pwscf.org/mailman/listinfo/pw_forum
>
--
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20171003/a513eb08/attachment.html>
More information about the users
mailing list