"The principle of the Monte Carlo procedure is very simple: If we define a box (volume is L) which is able to hold the entire system, and let N particles randomly distributed in the box, if n particles are presented in the vdW region, then the vdW volume of present system is n/N*L. Of course, the result improves with the increase of N. In Multiwfn, you need to define N by input a number i, the relationship is N=100*2i, for small molecular when i=9 the accuracy is generally acceptable, for large system you may need to increase i gradually until the result variation between i and i+1 is small enough to be acceptable as converged.

I cannot find the command/location for changing N by defining i. Please advise.

Also thank you for the timely response to my previous question. I have succeeded in using Multiwfn to do the computation I required and I will of course include the citation you request in any publication.  It's a very useful program!