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 <[email protected]>
wrote:
On Mon, Oct 2, 2017 at 4:25 PM, JAY Antoine <[email protected]>
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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