Dear sir:

Is it possible to calculate Fukui functions and Dual descriptor using HOMO and LUMO densities on Multiwfn?
Thanks in advance!

It is possible. For calculating Fukui function based on FMO approximation, you should properly set orbital occupation numbers and then calculate electron density. For example, f+ Fukui function simply corresponds to density of LUMO orbital. To plot its isosurface, you should load a wavefunction file containing unoccupied MOs (e.g. fch, molden), then input
6  // Modify wavefunction
26  // Set occupation number of some orbitals
0  // Select all orbitals
0  // Set occupation number to 0
20  // I assume LUMO corresponds to MO20
1  // Set occupation number to 1
q  // Return
-1  // Return to main menu
5  // Calculate grid data
1  // Electron density
2  // Medium quality grid
-1  // Visualize isosurface

Now the plotted isosurface corresponds to f+ under FMO approximation. You can also use corresponding option to export the grid data as cube file.

To calculate dual descriptor based on HOMO and LUMO densities, you need to use the above way to calculate and export two cubes file corresponding to LUMO density and HOMO density, respectively, and then use subfunction 11 of main function 13 to get their difference, see Section 4.13.2 of Multiwfn manual for example.

A much easier way of calculating them is following the example in Section 4.100.16.2 of Multiwfn manual. This example illustrates how to plot orbital-weighted Fukui function and orbital-weighted dual descriptor. In this interface, if you choose "4 Set delta in orbital-weighted (OW) calculation" option and input a very small value for delta parameter, then the orbital-weighted Fukui function and orbital-weighted dual descriptor will simply correspond to the Fukui function and dual descriptor under FMO approximation, respectively.

Is it possible to calculate Fukui functions and Dual descriptor using HOMO and LUMO densities on Multiwfn?
Thanks in advance!

I am trying to calculate fukui functions for a simple molecule. I followed the steps written in the Multifwn manual and tried to export the data to a .txt file by choosing number 3 from the menu:

-1 Show isosurface graph
0 Return to main menu
1 Save graph of isosurface to file in current folder
2 Export data to a Gaussian-type cube file in current folder
3 Export data to output.txt in current folder
4 Set the value of isosurface to be shown, current:   0.05000
5 Multiply all grid data by a factor
6 Divide all grid data by a factor
7 Add a value to all grid data
8 Substract a value from all grid data

And then I got this message:

Outputting output.txt in current folder...
Output finished, column 1/2/3/4 correspond to X/Y/Z/value. The coordinate unit is Angstrom, the unit of the calculated function is a.u.

But when I checked the folder for the .txt file, it is not there. Can someone tell me what could possibly be the issue? Thank you!