Can you help me to calculate delocalization index of QTAIM using multiwfn? I read the manual but i did not get the method.

The concept of delocalization index (DI) is detailedly described in Section 3.18.5 of the manual. In the original definition of DI, this quantity is calculated between two AIM basins, see "Calculate localization index and delocalization index in basins" of Section 4.17.1 for calculation example. The DI can also be calculated between other kinds of basins, for example, ELF basins, see Section 4.17.2 for example.

Multiwfn also supports calculating DI between "fuzzy atomic spaces", an example is given in Section 4.15.1. In J. Phys. Chem. A, 109, 9904 (2005) it is shown that this form of DI is equivalent to the so-called fuzzy bond order, which is described in Section 3.11.6 and can be calculated by means of subfunction 7 of main function 9.

It is also worth to note that the Mayer bond order is essentially DI between two atoms, but defined in Hilbert space. It can be calculated by subfunction 1 of main function 9, introduction can be found in Section 3.11.1.