Molecular dynamics (Mac OSX user): what software can I use to plot RDFs for NPT simulations? VMD only works for NVT (constant volume)
Thank you, Buck, for all your suggestions.  You're right that a good way to check the influence of the volume is to try varying it.  I did that by using the maximum and minimum volumes of the fluctuations in one simulation.  There was some variation, so the volume does matter!

I was lucky enough to hear about "travis," an amazingly simple-to-compile code (even for Macs) available online for free.  I thought I'd share that information here because other people are bound to run into this same problem.  This code, travis, takes my trajectory file and my volume file and then asks a number of questions interactively.  It outputs a nice CSV file that can be plotted.

Thanks again for your help, Buck.  Really helped me think this through.  I'd like to ask the moderators or whoever is in charge of tagging to please add some molecular dynamics-related tags if possible!  Thank you.