I think you can try looking up articles online, DFT papers that use a similar system to what you want to work on. Then you can choose a suitable method and basis-set you think it's appropriate. And note that the basis set you to choose also depends on the number of computer resources you have..

Thank you 🙏🏻

EDIT (12/13/2020):

If you /are working on big system such as MOFs and COFs, a commonly used method for large systems the ONIOM method, which consists of using more than one method/basis set. Usually a combination of low and medium(or high) level of theory to cut computational cost.

Check the following review article for more details:

Article The ONIOM method and its applications

You can also check Gaussian official page to see how to set up ONIOM calculations on Gaussian: https://gaussian.com/oniom/

ORIGINAL:

Hi! If you are working on a biomolecule, chances are your system is big. A commonly used method for large systems the ONIOM method, which consists of using more than one method/basis set. Usually a combination of low and medium(or high) level of theory to cut computational cost.

Check the following review article for more details:

Article QM/MM methods for biomolecular systems. Angew Chem Int Ed 48:1198

Article The ONIOM method and its applications

Domoina Holiharimanana Hi, Well I'm not working on biomolecules but the information you shared was really valuable, I appreciate it, Thank you

@ Hanieh Moradi I misread your post :( I edited my answer in case you're working on COFs and MOFs.

If you are working on small organic or organometallic molecules, DFT B3LYP functional is the go-to method. The Basis set can be a bit tricky for organometallic molecules because you may need to choose two different basis set for your organic atoms and transition metals. For example 6-31G(d,p) for your C, H, O etc and LANL2DZ for your TM.

For inorganic systems, PBE functional is commonly used.

I hope this help