Dear Shamik,
We can answer your question in many ways.
First of all, DFT is a theory of ground state properties. We thus do not
expect good simulations of excited states properties!
However, we are commonly using DFT to reproduce excited states
properties (optical, magnetic ...), while we must go beyond DFT (GW,
TDDFT, HF-CI, ...).
Thus we have to be pragmatic because we want to investigate properties
of solid state materials, which are usually based on large number of atoms.
Using specific functionals allow to describe a wide range of materials
and properties, but it requires a deep knowledge of all these
functionals (limitations ...).
Long time ago, researchers in solid state science were mainly using LDA
and GGA, knowing their limitations and thus knowing what to do with
these functionals and how to improve them when localized states and/or
strong correlation were present (LDA+U, GGA+U). Now, we have more
functionals and more to know.
In addition, you must be aware that mBJ is not a functional but a
semilocal potential. It means that using mBJ you cannot use the energy
to estimate the atomic forces for instance. However, using TB-mBJ you
can really obtain band gaps of the quality of GW calculations with the
price of a LDA calculation.
Pratically speaking, one today strategy is to use a functional for the
geometry optimization and if your system is too large for an hybrid
functional, use TB-mBJ for the estimation of the band gap, effective
masses ...
To conclude, there is no unique answer to your question but the more
important is to learn the subtle details beyond each functionals.
You will then see that you need only very few of them ... because the
more important is not to be exact but to be able to understand and
explain the relationship between the properties and the atomic structure
through the electronic structure.
Best regards
Xavier
Le 17/02/2020 à 07:44, shamik chakrabarti a écrit :
Dear Wien2k users & Experts,
I have a basic question regarding
simulation using different functionals. I have learned that simulation
using mbj would provide more accurate band gap than it is provided by
either GGA or GGA+U or nlvdw. On the contrary, if we want to check
accurate lattice parameters we go for rev-vdW-DF2 & again if we need
to check accurate cohesive energy we go for SCAN. So, for checking
different parameters we use different functionals while one property
with one functional may be correct & at the same time one property
with the same functional is not accurate enough.
Why is that so? Why there is no unique functional by using which we
can get all the properties relatively accurately.
Looking forward to your esteemed advices.
with regards,
--
Dr. Shamik Chakrabarti
Research Fellow
Department of Physics
Indian Institute of Technology Patna
Bihta-801103
Patna
Bihar, India
_______________________________________________
Wien mailing list
Wien@zeus.theochem.tuwien.ac.at
http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
SEARCH the MAILING-LIST at:
http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html
--
------------------------
Institut des Sciences Chimiques de Rennes (ISCR)
Univ Rennes - CNRS - UMR6226, France
https://iscr.univ-rennes1.fr/cti/people/permanent-staff/rocquefelte-xavier
------------------------
ICAMM2019 : VASP Workshop and International Materials Modelling Conf
June 26-July 3 2019, Rennes France
icamm2019.sciencesconf.org : Registration opening soon, see you there!
_______________________________________________
Wien mailing list
Wien@zeus.theochem.tuwien.ac.at
http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien
SEARCH the MAILING-LIST at:
http://www.mail-archive.com/wien@zeus.theochem.tuwien.ac.at/index.html
