mirror of https://gitlab.com/QEF/q-e.git
a0ae1121eb
1) Remove PP files from examples of TDDFPT, use an automatic download of PPs from the QE web site; 2) TDDFPT examples are now in the standard format of QE (using run_example scripts); 3) Fix the bug in TDDFPT+EXX due to recent changes in PW (fft_fact); 4) The ecutfock parameter in TDDFPT+EXX is now read from XML and cannot be specified from the input of TDDFPT; 5) In TDDFPT the oscillator strength (absorption coefficient) is now written in a separate file (and not in the same file where \chi is written); 6) HP examples use PPs from the Pslibrary by an automatic download from the QE website. |
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README
In order to compute U for magnetic insulators one needs to follow
special procedure. Since we do not know a priori the magnetization
of the system, we perform a calculation with a finite
starting_magnetization (in the range from -1 to +1) which requires
to treat the system of interest as a metal (i.e. to use a smearing).
However, using the smearing can create problems in the linear-response
calculation of U, because in DFPT for metals at q=0 there is a metallic
correction which is proportional to the inverse of the DOS at the Fermi
level [see Eq.(79) in Rev. Mod. Phys. 73, 515 (2001)]. Therefore, if we
treat the system as a fake metal, then the DOS at the Fermi level might
be extremely small, which would lead to extremely large shift of the
Fermi energy and as a consequence to the numerical divergence of the
DFPT calculation. The solution to this problem is the following:
1. Perform a SCF calculation by treating the system as a fake magnetic
metal. Namely, specify occupations = 'smearing' and set up some very
small broadening parameter, and set up nspin=2 and starting_magnetization.
Moreover, use the convergence threshold conv_thr not too small, because
otherwise there might be problems to converge the SCF calculation in
the next step.
2. Perform a SCF calculation by treating the system as a magnetic insulator
using the total magnetization obtained in the previous step. Extract from
the previous calculation the total magnetization and the number of bands
(Kohn-Sham states) and use them in this SCF calculation by using the
keywords tot_magnetization and nbnd, respectively. Some details:
- set up the occupations to be fixed (this is the default in QE), namely
occupations = 'fixed';
- use nspin=2 and tot_magnetization, where the total magnetization must
be extracted from the previous step. The setup of tot_magnetization
is needed in order to let the code specify correctly the occupations
for spin up and spin down electrons (without specifying
tot_magnetization the code will set the number of spin up and spin down
electrons to be equal, which is wrong for ferromagnetic insulators).
If tot_magnetization deviates slightly from the integer value then
round it to the integer value, because non-integer tot_magnetization cannot
be used in the second SCF calculation (the deviation from the integer
can be due to numerical noise);
- set up the number of bands (nbnd) equal to the number of bands in the
previous run [without specifying nbnd in this calculation, the code will
set it to be equal to half the number of electrons, while in the
previous (metallic) calculation the number of bands was determined as
half of the number of electrons plus 20% - this mismatch would lead
to numerical problems];
- in order to save CPU time in the current SCF calculation it is
recommended to start from the wavefunctions and the potential obtained
in the previous step. To do so specify in the input startingpot='file'
and startingwfc='file'. If you ignore this and try to start the SCF
calculation from scratch, it may happen that the code will not converge
at all (because it would be hard for it to find the ground state and
the correct magnetization starting just from the condition of the
constrained magnetization).
3. Perform the linear-response calculation of U using the HP code.