[gmx-users] g_hbond + invalid command line argument -g

[minnale]

Hi all, 
  I am new to gromacs, I am interested in calculate Hydrogen bonds of my protein. So I have issued the *g_hbond command like this
g_hbond -f .xtc -s .tpr -n .ndx(Index file contain all residues mainchain+H atoms) -num .xvg -g .log
its showed 

 Program g_hbond, VERSION 3.3.1
Source code file: statutil.c, line: 799

Invalid command line argument:
-g

Can you please tell where I have done mistake?
Thanks in advance for your valuable suggestions.
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