Hello, 1) I want to calculate the occupancy of water around few base pairs in a RNA duplex. I had one query while i was going through the "calculate atomic occupancy" documentation. "For example, cations or water hydrogens may tend to occupy space around a negatively charged solute group" In the options give for this particular calculation, nowhere there is an option to select whether i am interested in WATER or ION occupany. How then chimera calculates the occupancy. 2)I presume that chimera calculates the occupany of say water as follows, if i select a residue, it puts the residue in a grid and calculated the density of water molecule in the grid points in frame. Is this the way it calculates the occupancy. I am not getting how i should differentiate occupancy and density. thanks, Bala