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<p>Dear Shamik,</p>
<p>We can answer your question in many ways. <br>
</p>
<p>First of all, DFT is a theory of ground state properties. We thus
do not expect good simulations of excited states properties! <br>
</p>
<p>However, we are commonly using DFT to reproduce excited states
properties (optical, magnetic ...), while we must go beyond DFT
(GW, TDDFT, HF-CI, ...).<br>
</p>
<p>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. <br>
</p>
<p>Using specific functionals allow to describe a wide range of
materials and properties, but it requires a deep knowledge of all
these functionals (limitations ...). <br>
</p>
<p>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. <br>
</p>
<p>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. <br>
</p>
<p>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 ... <br>
</p>
<p>To conclude, there is no unique answer to your question but the
more important is to learn the subtle details beyond each
functionals. <br>
</p>
<p>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. <br>
</p>
<p>Best regards</p>
<p>Xavier <br>
</p>
<p><br>
</p>
<p><br>
</p>
<p>Le 17/02/2020 à 07:44, shamik chakrabarti a écrit :<br>
</p>
<blockquote type="cite"
cite="mid:CAA+63+rS8GzDP4z21RDMb2QDQwEb9JX5dsGa7+8x9FMmPf7moA@mail.gmail.com">
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<div dir="ltr">Dear Wien2k users & Experts,
<div><br>
</div>
<div> 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. </div>
<div><br>
</div>
<div>Why is that so? Why there is no unique functional by using
which we can get all the properties relatively accurately. </div>
<div><br>
</div>
<div>Looking forward to your esteemed advices.</div>
<div><br>
</div>
<div>with regards, <br clear="all">
<div><br>
</div>
-- <br>
<div dir="ltr" class="gmail_signature"
data-smartmail="gmail_signature">
<div dir="ltr">
<div>
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div dir="ltr">
<div style="font-size:small">Dr. Shamik
Chakrabarti</div>
<div style="font-size:small">Research Fellow </div>
<div style="font-size:small">Department of
Physics</div>
<div style="font-size:small">Indian Institute of
Technology Patna</div>
<div style="font-size:small">Bihta-801103</div>
<div style="font-size:small">Patna</div>
<div style="font-size:small">Bihar, India</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
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<br>
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