Hi, when I run pdb2gmx -v -f 2KO3.pdb -o initial-vacuum.gro -ff oplsaa -water tip4p -vsite hydrogens -ignh , I got Fatal error: Invalid directive HISD in vsite database /home/XXX/gromacs/g/share/gromacs/top/oplsaa.ff/aminoacids.vsd . What is the problem? 2KO3.pdb was downloaded from PDB. Best regards, Ting