I'm performing molecular dynamics of a metalloprotein containing a Mg2+ ion with some surrounding negatively charged amminoacids with Desmond. When i run the simulation preparing the system with system builder (OPLS_2005) everything is fine but then when I try to convert it with Viparr utility with other forcefields like Charmm27 or others I keep getting errors for the Mg ion and for the charged amminoacids around it. Anyone knows how to solve it?