[gmx-users] Scd or -Scd?

Thomas Piggot t.piggot at soton.ac.uk
Fri Jul 14 19:46:18 CEST 2017

As far as I'm aware (and happy to be corrected if someone knows better) 
it is simply a convention that stems from the fact that the signs of the 
order parameters were not originally determined from experiments and so 
were reported in these works as |Scd|. Given that the order parameters 
of the lipid tails are pretty much always negative, it makes sense to 
show -Scd to keep the plots looking the same as those that report the 
absolute values of the Scd.

That said, you should be careful with reporting the absolute values 
though as the order parameters (Scd) can theoretically vary between -0.5 
and 1. Indeed with some force fields (e.g. ones with unsaturated double 
bonds) you can have order parameters that do cross 0 (i.e. will be 
plotted incorrectly if you report [Scd]).


Tom (not Justine!)

On 14/07/17 18:01, m g wrote:
> Dear Justine,Why are the order parameter data reported in articles as -S_CD (minus S_CD)?
> Thanks,Ganj

Dr Thomas Piggot
Visiting Fellow
University of Southampton, UK.

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