[gmx-users] sn1 and sn2 chains of popc

[minnale]

Hi all, 
may be this is very basic query
I want to calculate order parameters of popc. I have found in archives that make sn1.ndx(pamitoyl) and sn2.ndx(oleyl) of popc then feed these files to g_order command. Can anyone tell me that why pamitoyl call as sn1 and oleyl call as sn2?
Thanks for your help.
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