[QE-users] How to decide value of TDDFPT variables

[Timrov Iurii]

Dear M. Inukai,

1. num_eign = number of eigenstates (excitations) which will be
computed. This parameter must be chosen based on how large energy range
for the spectrum the user is interested in. The bigger the energy range,
the larger num_eign is. A priori it is not possible to guess this
number, because it depends on the system. Try with some reasonable
numbers (say, 5, 10, or 15) and then check whether this is enough to
obtain a converged spectrum in the energy range of interest.

2. 'num_basis_max' is the maximum number of basis allowed in the subspace.
In  every Davidson iteration the number of basis used in the subspace is increased
by num_init. When num_basis_max is reached, a discharging
routine is called (this variable determines the memory requirement). The
larger the parameter num_eign, the larger num_basis_max would be needed.
Try with some reasonable numbers, run test calculations, and then
increase/decrease num_basis_max based on your observations. Use examples
in TDDFPT to get an idea about typical values of this parameter.

3. 'ecutfock' is the kinetic energy cutoff (Ry) for the exact exchange operator
in the exact-exchange (EXX) type calculations. The meaning of this parameter
is the same as in the PW code (pw.x), hence see the documentation in PW/Doc.
The value of ecutfock cannot be smaller than ecutwfc.

4. 'itermax0' generally you can use the same number that you computed
with turbo_laczos (itermax).

5. 'p_nbnd_occ and p_nbnd_virt' if you are interested only in energy and
absorption spectrum, it is not necessary to use all occupied and virtual
orbitals. Using all states is useful only if you are interested in the
interpretation of the eigenstates.

6. 'pseudo_hermitian' When you use .FALSE., the code will be 2 times
slower but the result must be the same. So it is always recommended to
use .TRUE. unless it does not work for some reasons. The .FALSE. option
was implemented first, and hence it is still kept in the code for the
case when .TRUE. does not work.

7. 'CH4-BOND' this is obsolete and will be removed in the next release of QE.
The meaning of "project", "absorbtive", and "dispersive" is not documented,
it was introduced by Osman Baris Malcioglu around ~2011 (CPC 182, 1744 (2011)).
These parameters are related to the calculation of the response charge density.
You may try either to understand what does these keywords mean by looking into the code or
try to contact Baris and ask him directly.

Best regards,
Oscar Baseggio - postdoctoral researcher, SISSA, Trieste, Italy
Iurii Timrov - postdoctoral researcher, EPFL STI IMX THEOS, Lausanne, Switzerland


________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of 犬飼　学 <inukai.manabu at nitech.ac.jp>
Sent: Monday, November 26, 2018 2:17:37 AM
To: users at lists.quantum-espresso.org
Subject: [QE-users] How to decide value of TDDFPT variables

Dear developers

Although I have red *.html in Doc (TDDFPT),
I do not understand same variables in detail.

Would you tell me any advices ?
----------
How do I decide 'num_eign', 'num_basis_max' and 'ecutfock' in *.dav-in ?

How do I decide 'itermax0' in *.tddfpt_pp-in ?

I set number of eigenvalue (<= EF) to p_nbnd_occ and
set number of eigenvalue (>= EF) to p_nbnd_virt ?
Is it best ?

What do absorbtive and dispersive mean in CH4-BOND ?

What does 'project' mean in CH4-BOND ?

When do I use pseudo_hermitian=.false.
--------
Best Regards,
M. Inukai

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