Surprising default combining rule in association term

[ianhbell]

After quite some investigation my colleague and I figured out what was going on in the association implementation in Clapeyron and we were quite surprised to find that the default combining rule is no(!) combining rule. From my side, it seems like CR1 should probably be the default. I figured out eventually how to make the combining rule be CR1, which is not terribly well documented.

using Clapeyron
options = AssocOptions(combining=:cr1)
model = Clapeyron.CPA(["Ethanol", "Water"], assoc_options=options)

[pw0908]

On the documentation, admittedly, it's not in the most obvious place, but we did document the fact that we do have no combining rule is the default: https://clapeyronthermo.github.io/Clapeyron.jl/dev/api/parameters/#Clapeyron.AssocOptions.

As for what the default should be, the difficulty is that did SAFT equations use different combining rules (I believe that SAFT-VR Mie uses Elliott whereas CPA using CR1. When it comes to specifying association parameters though, I do believe that it is better to leave the combining rule off by default as, when you get to SAFT-VR Mie / gamma Mie where you have H, e1, e2 and a1 association sites where you don't necessarily expect cross-association between everything, it's safer to leave it up to the user to specify.

I will add some docs on that in the documentation that I'm updating.

[pw0908]

Happy to report the updated docs do explicitly highlight how the association interactions can be adjusted: https://github.com/ClapeyronThermo/Clapeyron.jl/blob/docs-update/docs/src/tutorials/basics_model_construction.md

[ianhbell]

If you explicitly select CPA/sCPA, I feel like the default sent to the association term should be CR1. Or at least, it should be clearly documented in the model documentation for all the relevant models that the combining rule is not set at all. I see your point about the nuances of how the association term gets used in other places, and sympathize. I'm struggling with the same thing in teqp now.