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pw.x determines first the symmetry operations (rotations) of the
Bravais lattice; then checks which of these are symmetry operations of
the system (including if needed fractional translations). This is done
by rotating (and translating if needed) the atoms in the unit cell and
verifying if the rotated unit cell coincides with the original one.
Assuming that your coordinates are correct (please carefully check!),
you may not find all the symmetries you expect because:
- the number of significant figures in the atomic positions is not
large enough. In file PW/eqvect.f90, the variable accep is used to
decide whether a rotation is a symmetry operation. Its current value
(10-5
) is quite strict: a rotated atom must coincide with
another atom to 5 significant digits. You may change the value of
accep and recompile.
- they are not acceptable symmetry operations of the Bravais
lattice. This is the case for C60
, for instance: the Ih
icosahedral group of C60
contains 5-fold rotations that are
incompatible with translation symmetry.
- the system is rotated with respect to symmetry axis. For
instance: a C60
molecule in the fcc lattice will have 24
symmetry operations (Th
group) only if the double bond is
aligned along one of the crystal axis; if C60
is rotated
in some arbitrary way, pw.x may not find any symmetry, apart from
inversion.
- they contain a fractional translation that is incompatible with
the FFT grid (see next paragraph). Note that if you change cutoff or
unit cell volume, the automatically computed FFT grid changes, and
this may explain changes in symmetry (and in the number of k-points
as a consequence) for no apparent good reason (only if you have
fractional translations in the system, though).
- a fractional translation, without rotation, is a symmetry
operation of the system. This means that the cell is actually a
supercell. In this case, all symmetry operations containing
fractional translations are disabled. The reason is that in this
rather exotic case there is no simple way to select those symmetry
operations forming a true group, in the mathematical sense of the
term.
Next: 7.21 warning: 'symmetry operation
Up: 7 Troubleshooting
Previous: 7.19 the FFT grids
Contents
Paolo Giannozzi
2010-04-08