Dear Vaibhav Sachdeva ,

There are many things to take into account for choosing a functional (and a basis-set):

- What do you want to study? Do you only want to see orbitals and their relative energies? Are you planning to make transition-state/excited-states/NMR etc... Calculations?

- If you don't know about the functional, you may find information that has been published with this kind of system.

- If it's new, generally, you should do a benchmark of functionals and basis-sets to be as close as possible to reported experimental results.

- If not experimentally reported, you should perform a heavy calculation (post-HF) on which you can rely on to choose a proper functional. I also sometimes take a look on the little table I put in my answer to adjust my choice (sorry, I don't remember from where I took this table years ago...).

I don't know what you want to do, but generally, for this kind of molecules, I'm used to take M062X functionnal and Def2-TZVP as basis-set. Of course, choice of functional will depend on the study you want to do.

I hope my little answer could help you!

Corentin Lefebvre , Thank you for such a detailed answer. I am working to find why Pyrrole shows different dipole moment direction as compared to the direction showed by Thiophene and Furan , so using B3LYP As exchange correlation and 6-31G** basis set to solve the problem. It gives accurate results for Pyrrole and furan but not for Thiophene that's why i am confused and wanted to know which exchange correlation and basis set duo to be taken for better results

Thank you Dr. C. Lefebvre for the wealthy information provided.

@CorentinLefebvre