I guess the reason is that the conversion from cartesian to crystal axis
for G-vectors is done in-place using a routine unsurprisingly called
"cryst_to_cart". The output is in the same real variable "g0". It is not
wise to compare real numbers with "if (a == b)", as one would do with
integers, even if in practice they are integers.
Paolo
On 3/14/2026 2:21 PM, Roland Winkler wrote:
Dear Paolo,
now that Stefano corrected my embarrassing mistake regarding the space
group symmetry of wurtzite (that explains everything I am seeing),
I kindly would like to ask one more technical question that you may be
able to answer, as you said you wrote the code for the charge
symmetrization.
I want to understand the generic approach implemented in the code for
charge symmetrization. It seems to me that the initialization performed
by sym_rho_init_shells is conceptually very similar to the actual
symmetrization performed by sym_rho_serial. Both routines map
reciprocal lattice vectors onto their images under the crystallographic
point group. (The phase factors associated with nonsymmorphic
symmetries only affect the images of the charge density and
magnetization.)
Was there a reason why you implemented the rotations of the reciprocal
lattice vectors in sym_rho_init_shells via integer arithmetic, but
sym_rho_serial uses floating point arithmetic for this task? It would
seem to me that integer arithmetic should work in both cases and it
would be better suited for this task than floating point arithmetic.
Thank you,
Roland
On Sat, Mar 14 2026, Paolo Giannozzi wrote:
I wrote many years ago the code that symmetrizes the charge density
in reciprocal space. Unfortunately I have no time right now to delve
into your question, but please note that the code is generic, uses
the available crystal symmetry and knows nothing about your specific
case.
In the first versions of QE symmetry operations were stored as
matrices of integer numbers, acting on crystal axis. This is
practical to deal with transformation of real-space grid
indices. For all other cases, symmetry operations are transformed to
real matrices, acting on cartesian coordinates.
PG
On 3/12/2026 7:20 PM, Roland Winkler wrote:
Dear Users
I am trying to understand how quantum espresso (pw) is
symmetrizing
the charge density rhog in reciprocal space. My understanding is
that for a nonmagnetic symmorphic crystal structure all components
of rhog in a star should be equal [Streitwolf, Group Theory in
Physics, Eq. (6.8)].
For systems with zincblende structure this is, indeed, what I get.
However, for systems with wurtzite structure half of the components
in some stars have opposite signs.
More specifically, I have checked that sym_rho_init_shells
identifies the stars as expected. However, sym_rho_serial then
gives opposite signs for half of the components in some stars.
The code in sym_rho_init_shells is quite straightforward, using
integer arithmetic for the transformations of the reciprocal lattice
vectors. This code is doing what I expect, and it gives the results
I expect. However, I have not yet got the same level of
understanding for sym_rho_serial. The latter routine is doing
something more complicated, implementing the transformations of the
reciprocal lattice vectors via floating point arithmetic where I do
not understand the reason for this different approach. I can merely
say that I have instrumented sym_rho_serial to write out the density
after symmetrization and I do not get what I expect as described
above. [For example, for the job attached below, the components
12,14,16,18,20,22 and 13,15,17,19,21,23 of rhog form two stars
(according to sym_rho_init_shells), but the corresponding values of
rhog in each of these stars are not equal.]
Am I missing something? Any help is appreciated.
My input file for pw.x is attached. I am using version 7.5.
Roland Winkler
&CONTROL
calculation='scf'
verbosity='high'
prefix='zns' ! Output files are named according to prefix
/
&SYSTEM
space_group=186 ! Space group number
a=3.811 ! Lattice parameter a in angstroms
c=6.234 ! Lattice parameter c in angstroms
nat=2 ! Number of atoms in the asymmetric unit
ntyp=2 ! Number of different atom types. Here, Zn and S.
ecutwfc=40 ! Kinetic energy cutoff for wavefunctions (Ry)
ecutrho=200 ! Kinetic energy cutoff for charge density and potential (Ry)
/
&ELECTRONS
/
&IONS
/
&CELL
/
ATOMIC_SPECIES
Zn 65.38 zn_pbe_v1.uspp.F.UPF
S 32.065 s_pbe_v1.4.uspp.F.UPF
ATOMIC_POSITIONS crystal_sg
Zn 1/3 2/3 0.00000
S 1/3 2/3 0.38500
K_POINTS automatic
2 2 2 0 0 0
_______________________________________________________________________________
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civilians worldwide who are victims of terrorism, military
aggression, and indiscriminate warfare.
--------------------------------------------------------------------------------
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--
Paolo Giannozzi, Dip. Scienze Matematiche Informatiche e Fisiche,
Univ. Udine, via delle Scienze 206, 33100 Udine Italy, +39-0432-558216
_______________________________________________________________________________
The Quantum ESPRESSO Foundation stands in solidarity with all civilians
worldwide who are victims of terrorism, military aggression, and indiscriminate
warfare.
--------------------------------------------------------------------------------
Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
users mailing list [email protected]
https://lists.quantum-espresso.org/mailman/listinfo/users
