[gmx-users] K+ ions in gromos53a6 FF ?

[Sim gmx]

Hi,

It seems well established that Na+ ions have a condensing effect on Berger
lipid bilayers, see for instance
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3000517/
To circumvent this inconvenience, it is advised to use larger cations, such
as K+ or TMA. It appears to me that K+ should be easier to use and to
justify (I might already be wrong here, feel free to correct me if
necessary). However, I just discovered that K+ ions were not present by
default in gromos53a6 FF.

Could please someone tell me why ? and which non bonded parameters should I
add for K+ ions ?
I found c6 and c12 values here http://pubs.acs.org/doi/abs/10.1021/ct9000763
but I'm not sure if it is totally reliable. Does anyone have other sources ?

Thank you in advance !