gmx order is for alkyl chains, e.g. deuterium order parameters of lipids. The tool you want is gmx hydorder, though the output may indeed need some additional parsing or modification of the underlying code to make it useful.

Thanks a lot for the information. Can you please suggest me what kind of modification is required for gmx_hydorder output?

You said the output was confusing. I suppose it's up to you how you'd like to represent the data if the default output is not useful to you, and therefore the necessary modifications.

Justin Lemkul The output of gmx_hydorder is a .xpm file, which is hard to interprete for me. Can you suggest how to generate a data file consist of relative occurrence vs tetrahedral order parameter at a particular time point, using gmx_hydorder?