It seems that the pka plugin gives pka values for an ionic strength of .10M (from comparing values to a table). Is there any way to get the pka for a different ionic strength, namely 0M?
It is nice that you managed to find such a good relation between predicted pKa and ionic strength.
pKa plugin can only calculate practical pKa values. Avarage standard deviation of our pKa prediction is between 1.0 - 0.2 pKa unit (depends on the molecule).
Ionic strength is not included explicitly in our pKa calculation.
Training set of pKa plugin containes both thermodynamic (I=0) and practical (I>0) pKa values.
Approximate relation derived between thermodynamic and practical pKa by Perrin et .al.:
‘pKa prediction for organic acids and bases, Chapman and Hall Ltd. 1981’
I hope it is help for you.