[gmx-users] RDF of hydroxyl groups

[Magdalena Siwko]

Dear All,

I'm working on simulation of mixed bilayer (DMPC and some kind of phenolic
lipid). My lipid in head has aromatic ring with two hydroxyl groups.
I want to know the change in hydratation of this mixed bilayer via function of
concentration of inserted lipids and tails lenght. 
I calculated rdf in two ways.
1. For 2 hydrogens of phenolic lipid hydroxyl groups (as a one group in index 
file - H5_H13) - reference group and OW (oxygen in water), OA-P-OB-OC-OD
(phosphate group of DMPC) and also for two carbonyl groups OE-C1A-Of and 
OG-C2A-OH.
2. Reference group OW (solvent) and hydrogens from hydroxyl groups and phosphate
and carbonyl groups as above. This set-up looks reasonable.

I wonder why in first set-up rdf for one carbonyl group OE-C1A-OF is so 
large - 44 where usually in papers this values are around 2-4?
Is the correct way I define groups in index file?

Pls, some hints.

Thank you,
Magda