It is impossible. You can only use Multiwfn to convert .fchk to .wfn file. (.wfn file doesn't carry basis function information like .fchk)
dear multiwfn users
I want to know if
is there someway to convert a file .wfn to .fchk?
because after that i want to use the fchk to do a single point with hartree fock in gaussian