Unless otherwise specified, all PWscf input and output quantities are in atomic "Rydberg" units, i.e. energies in Ry, lengths in Bohr radii, etc.. Note that CP uses instead atomic "Hartree" units: energies in Ha, lengths in Bohr radii.
In most cases: your input data is bad, or else your system is metallic
and you are treating it as an insulator. If this is not the case:
reduce mixing_beta to
The total magnetization is the integral of the magnetization
in the cell:
There is no 'right' way of defining the local magnetic moment
around an atom in a multi-atom system. However an approximate way to define
it is via the projected density of states on the atomic orbitals (code
projwfc.x, see example08 for its use as a postprocessing tool). This
code generate many files with the density of states projected on each
atomic wavefunction of each atom and a BIG amount of data on the
standard output, the last few lines of which contain the decomposition
of Lowdin charges on angular momentum and spin component of each atom.
See input data documentation for projwfc.x.
"Lowdin charges (as well as other conventional atomic charges) do not
satisfy any sum rule. You can easily convince yourself that this is the
case because the atomic orbitals that are used to calculate them are
arbitrary to some extent. If you like, you can think that the missing
charge is "delocalized" or "bonding" charge, but this would be another
way of naming the conventional (to some extent) character of Löwdin
charge." (Stefano Baroni, Sept. 2008).
See also the definition of "spilling parameter": Sanchez-Portal et
al., Sol. State Commun. 95, 685 (1995). The spilling parameter
measures the ability of the basis provided by the pseudo-atomic wfc to
represent the PW eigenstates, by measuring how much of the subspace of
the Hamiltonian eigenstates falls outside the subspace spanned by the
atomic basis.
It is printed in the output. If not, the information on Gaussian smearing,
needed to calculate a sensible Fermi energy, was not provided in input.
In this case, pw.x prints instead the highest occupied and lowest
unoccupied levels. If not, the number of bands to be calculated was not
provided in input and pw.x calculates occupied bands only.
The reference level is an ill-defined quantity in calculations
in solids with periodic boundary conditions. Absolute values of
Kohn-Sham eigenvalues are meaningless.
"The value of the Fermi energy (as well as of any energy, for that
matter) depends of the reference level. What you are referring to is
probably the "Fermi energy referred to the vacuum level" (i.e.
the work function). In order to obtain that, you need to know what the
vacuum level is, which cannot be said from a bulk calculation only"
(Stefano Baroni, Sept. 2008).
More often than not, this charges are a slightly too hard to be
expanded very accurately in PWs, hence some aliasing error
will be introduced. Especially if the unit cell is big and mostly
empty, some local low negative charge density will be produced.
''This is NOT harmful at all, the negative charge density is handled
properly by the code and will disappear during the self-consistent
cycles'', but if it is very high (let's say more than 0.001*number of
electrons) it may be a symptom that your charge density cutoff is too
low. (L. Paulatto - November 2008)
Work function = (average potential in the vacuum) - (Fermi
Energy). The former is estimated in a supercell with the slab
geometry, by looking at the average of the electrostatic potential
(typically without the XC part). See the example in
examples/WorkFct_example.
0.3 ÷ 0.1
12.7.0.3 What is the difference between total and absolute magnetization?
(nup - ndown)d3r.
| nup - ndown| d3r.
12.7.0.4 How can I calculate magnetic moments for each atom?
12.7.0.5 What is the order of Ylm
12.7.0.6 Why is the sum of partial Lowdin charges not equal to
the total charge?
12.7.0.7 I cannot find the Fermi energy, where is it?
12.7.0.8 What is the reference level for Kohn-Sham energies?
Why do I get positive values for Kohn-Sham levels?
12.7.0.9 Why do I get a strange value of the Fermi energy?
12.7.0.10 Why I don't get zero pressure/stress at equilibrium?
If you make a calculation with fixed cell parameters, you
will never get exactly zero pressure/stress, unless you use the cell
that yields perfect equilibrium for your pseudopotentials, cutoffs,
k-points, etc.. Such cell will anyway be slightly different from the
experimental one. Note however that pressures/stresses in the order of
a few KBar correspond to very small differences in terms of lattice parameters.
12.7.0.11 Why do I get different results from vc-relax and from scf
on the same structure?
First of all, you should verify that the structure is really the same
(hint: compare Ewald energies). Also note that
that: a) the modified kinetic energy functional (often used in
variable-cell calculations) affects the calculated pressure/stress;
b) the PW basis set used in a variable-cell calculations is
determined by the cutoff and the initial cell gemometry.
If you make a calculation with the final geometry at the same
cutoff, you get slightly different results. The difference should
be small, though, unless you are using a too low cutoff for your
system. Since v.4.3.1, a final scf is performed at the end of the
vc-relax run to check for this.
12.7.0.12 Why do I get negative starting charge?
Self-consistency requires an initial guess for the charge density in
order to bootstrap the iterative algorithm. This first guess is
usually built from a superposition of atomic charges, constructed from
pseudopotential data.
12.7.0.13 How do I calculate the work function?
Next: 12.8 Phonons
Up: 12 Frequently Asked Questions
Previous: 12.6 Frequent errors during
Contents
Paolo Giannozzi
2011-07-17