Hello, I am intested in O-H---CL interaction in my system. I tried to use contact and -da together with following command g_hbond -f 3.trr -s 3.tpr -n hbond19.ndx -nonitacc -noda -r 0.5 -contact -num I am getting following error. Fatal error:Can not analyze contact between H and A: turn off -noda Nilesh