This depends on how to use Multiwfn. If you follows "Skill 1: Accelerating calculation of TrEsp by making use of cubegen utility" in Section 4.A.9 of Multiwfn manual, then you can use .fch/fchk as input file of Multiwfn, as shown in the manual (this example utilized cubegen, but it is not absolutely necessary)

Hi,

The manual specifies the use of .wfn files for calculating the TrESP following electronic structure calculations using Gaussian, however, is it possible to just use .fchk files and still get meaningful results? The Multiwfn program does not reject the use of .fchk files for this calculation but wanted to check if this is permitted?

Thanks