Hi, everyone:<br>I'm new in GROMACS. Currently I'm trying to build a lipid bilayer system with 40% POPC and 60% CHOLESTEROL, solvated in water. I got the 128 popc pdb file from Peter Tielemen's website. And in the literature I get that I should replace POPC with CHOL at some proper position in order to get the right pdb file, but I don't know how to do that. Can someone help me. Thanks a lot.