Hello, I would like to use OPC water model for my water simulations in Materials Studio ( see http://pubs.acs.org/doi/abs/10.1021/jz501780a ). This is rigid 4 point water model with 1 dummy atom. I created the given model H2O molecule, I created custom force field (see attached file). I was able also to use Amorfous Cell to create box filled with 500 of these molecules, but when I tried MD using Forcite (in Materials Studio 8.0)  I obtained error "Dynamics cannot integrate equation of motion for particles with a zero (or small mass.)". It seems that even if Materials Studio "know" dummy atoms there is not possible to use molecular systems containing dummy atom/s for molecular dynamic simulations (even when using quite recet MS version 8.0). Am I right ? If yes, is there any particle type which could substitute dummy atom in this case or some another trick which could allow to use in MS e.g. 4 or 5 point rigid water models for molecular dynamics studies ? Obviously I would rather prefer not to use the most trivial trick consisting in substitution of the dummy atom with hydrogen atom. Thank you in advance for relevant comments.