Jim

It is a bug, thank you for reporting!
I have updated version 3.6(dev) on Multiwfn website to solve this problem, please update your version.

Best regards,

Tian

If you would like to evaluate delta f in its exact form (based on rho of three states), you can follow below steps:
examples\phenol-1.wfn // N+1 electron system
5
0 // Set custom operation
3 // Three files will be operated with the file that has been loaded
-,examples\phenol.wfn // N electron system
-,examples\phenol.wfn // N electron system
+,examples\phenol+1.wfn // N-1 electron system

However, when I try this using the 3.5 April 6, 2018 released version on Windows 7 Pro, I get an error message.  Here is the output:

D:\Amber_DNA_QMMM\AAA_Multiwfn>Multiwfn
Multiwfn -- A Multifunctional Wavefunction Analyzer (for Windows 64bit)
Version 3.5, release date: 2018-Apr-6
Project leader: Tian Lu (Beijing Kein Research Center for Natural Sciences)
Below paper *MUST BE CITED* if Multiwfn is utilized in your work:
          Tian Lu, Feiwu Chen, J. Comput. Chem. 33, 580-592 (2012)
Multiwfn official website: http://sobereva.com/multiwfn
Multiwfn English forum: http://sobereva.com/wfnbbs
Multiwfn Chinese forum: http://bbs.keinsci.com/wfn
( The number of threads: 20   Current date: 2018-04-16   Time: 14:13:56 )

Input file path, for example E:\Kan_Colle\Kongou.wfn
(Supported: .wfn/.wfx/.fch/.molden/.31/.chg/.pdb/.xyz/.mol/.cub/.grd, etc.)
Hint: Press ENTER button directly can select file in a GUI window. To reload the file last time used, simply input the letter "
o". Input such as ?miku.fch can open the miku.fch in the same folder as the file last time used.
phenol-1.wfn
Please wait...

System energy:    -305.450018135556 Hartree,   Virial ratio:  1.99932966
Total/Alpha/Beta electrons:     51.0000     26.0000     25.0000
Net charge:    -1.00000    Expected multiplicity:    2
The number of orbitals:    51,  Atoms:     13,  GTFs:    220
This is unrestricted single-determinant wavefunction
Orbitals from 1 to    26 are alpha type, from    27 to    51 are beta type
Title line of this file:  hf/6-31G* opt=tight

Loaded phenol-1.wfn successfully!
Formula: H6 C6 O1
Molecule weight:        94.11124

                    ------------ Main function menu ------------
0 Show molecular structure and view orbitals
1 Output all properties at a point
2 Topology analysis
3 Output and plot specific property in a line
4 Output and plot specific property in a plane
5 Output and plot specific property within a spatial region (calc. grid data)
6 Check & modify wavefunction
7 Population analysis and atomic charges
8 Orbital composition analysis
9 Bond order analysis
10 Plot Total/Partial/Overlap population density-of-states (DOS)
11 Plot IR/Raman/UV-Vis/ECD/VCD spectrum
12 Quantitative analysis of molecular surface
13 Process grid data (No grid data is presented currently)
14 Adaptive natural density partitioning (AdNDP) analysis
15 Fuzzy atomic space analysis
16 Charge decomposition analysis (CDA) and extended CDA (ECDA)
17 Basin analysis
18 Electron excitation analysis
19 Orbital localization analysis
20 Visual study of weak interaction
100 Other functions (Part1)        200 Other functions (Part2)
5
-10 Return to main menu
-2 Obtain of deformation property
-1 Obtain of promolecule property
0 Set custom operation
             ----------- Avaliable real space functions -----------
1 Electron density                 2 Gradient norm of electron density
3 Laplacian of electron density    4 Value of orbital wavefunction
5 Electron spin density
6 Hamiltonian kinetic energy density K(r)
7 Lagrangian kinetic energy density G(r)
8 Electrostatic potential from nuclear charges
9 Electron Localization Function (ELF)
10 Localized orbital locator (LOL)
11 Local information entropy
12 Total electrostatic potential (ESP)
13 Reduced density gradient (RDG)     14 RDG with promolecular approximation
15 Sign(lambda2)*rho    16 Sign(lambda2)*rho with promolecular approximation
17 Correlation hole for alpha, ref. point:   0.00000   0.00000   0.00000
18 Average local ionization energy
19 Source function, mode: 1, ref. point:   0.00000   0.00000   0.00000
20 Electron delocalization range function EDR(r;d)
21 Orbital overlap distance function D(r)
22 Delta_g function
100 User-defined real space function, iuserfunc=    0
0
How many files to deal with? (Excluding the file that has been loaded)
3
Avaliable operators: +,-,*,/
e.g. -,sob.wfn means subtracting property of sob.wfn from the first file
Input operator and file path of system    1
-,phenol.wfn
Input operator and file path of system    2
-,phenol.wfn
Input operator and file path of system    3
+,phenol+1.wfn
Error: You didn't properly specify operator, re-input again!

I cannot get the "+" operator to work.  What do I need to do to get this to work properly?

Thanks.