There is no corresponding function in Multiwfn. Calculating Boltzmann population is quite easy, I provided a Excel file (//www.umsyar.com/attach/165/Boltzmann.xls) in my blog article //www.umsyar.com/165 for this purpose. You just need to manually input relative Gibbs free energy and set temperature. There are 8 data in default in the table, you can remove some of them or add additional data. Note that the relative Gibbs free energies should be calculated at the same temperature as the temperature value you set in the table.

In addition, you can use my code Shermo (//www.umsyar.com/soft/shermo/) to calculate Boltzmann population directly based on output file of freq or opt+freq task of many quantum chemistry programs, it is even more convenient than using the Excel table. Please search "Boltzmann" in Shermo manual.

Is there any options available within Multiwfn to calculate the Boltzmann Population of isomers from their free energy changes? If not available within Multiwfn, what other alternates are there to calculate it?