Which kations you use to neutralize your system in GROMACS (Na+ or K+)?

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GROMACS version: 2018.4
GROMACS modification: No

For anions it is simple, always CL-. For cations there is some options to choose. For example in charmm-gui membrane builder CHARMM-GUI the default option is K+, but in many GROMACS threads I saw that people use also Na+. Is there be any huge difference if you neutralize your system with a different ion or it doesn’t matter? I mostly work on membranes, so I was curious that is different ions can have an influence on for example membrane properties like area per lipid or number of hydrogen bonds or lipid etc.

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It depends on the quality of the force field in terms of ion interactions with the phosphate head groups. There are specific Na+:lipid NBFIXes in CHARMM that have been validated against osmotic pressure data. I’m not sure if K+ have, but it is a global default in CHARMM-GUI for the Solution Builder, Membrane Builder, etc. Not sure why, honestly.

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@jalemkul Thank you so much.