I want to convert .res file to .cif file. Which software tool will be best for this?
I have searched the web and found that PLATON might be able to help you. The WinGX application doesn't have support for RES and CIF file. I recommend you access the page provided below to see exactly how the software works and how to install it.
Home page: http://www.cryst.chem.uu.nl/spek/platon/pl000000.html