Implementation of MBIS in Multiwfn has been updated today, making it perfect:

MBIS atomic space now is available in fuzzy analysis module (choosing option -1 and select it in main function 15).

Calculation of MBIS charges has supported elements heaver than Ar (up to Rn currently), and the case of employing pseudopotential has been supported.

Population and width of shells (represented by Slater funtions) now can be outputted (in the MBIS charge calculation interface, choose option -2 once) before calculation.