Please explain to me in details. I can't understand corresponding contents in manual.
You cannot convert gjf files to wfn files: gjf files contain only the input information, wfn files contain some of the output information.
As to the chk file: I guess that you intend to do QTAIM analysis on the file. Many softwares for AIM (eg: the wonderful and free Multiwfn) let you use a chk file (or a fchk file) as input. I don't know if they can save a wfn file from that, but the AIM analysis can be performed anyway.
If you specifically need the wfn file, you can generate it with Gaussian:
- In a new calculation, by specifying the keyword output=wfn and, after the coordinates, filename.wfn. See Dr. Barroso's blog for nice examples, eg: https://joaquinbarroso.com/2016/02/16/quantifying-%CF%83-holes-g09-and-multiwfn/
- For an already performed calculation (for which a *.chk file is available) you can do the same; adding geom=allcheck option, the calculation uses the geometry and results in the already converged *.chk, and also outputs a wfn file
Some other caveats are listed here: http://aim.tkgristmill.com/knownissues.html (for instance, for the particular cases in which DENSITY=CURRENT, POP=NOAB, etc. are needed)
A couple of examples I wrote ("!" is seen as a comment line by Gaussian, I wrote it to put the blank line at the end of the code on stack exchange).
Optimization of HF molecule, with generation of both a *.chk file and a *.wfn file:
%chk=HF.chk #n output=wfn B3LYP/6-31G(d) Opt Hydrogen fluoride 0 1 F 0.09138 0.00000 0.00000 H -0.84046 0.00000 0.00000 HF.wfn !
Now, let's assume that the calculation was run without generation of wfn file (you have a *.chk file and you want a *wfn file):
%chk=HF.chk #n output=wfn B3LYP/6-31G(d) geom=allcheck guess=only HF.wfn !
%chk=HF.chk #n output=wfn B3LYP/6-31G(d) geom=check guess=(read,only) Hydrogen Fluoride 0 1 HF.wfn !