[Wien] A basic question

[shamik chakrabarti]

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
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