2,2,4-trimethylhexane (the challenge compound)

[ramess101]

@Kaiveria @mostafa-razavi @jrelliottoh

OK, now that we have finalized our manuscript for the special issue of FPE, it is time to move our attention to the challenge compound. The current plan is to simulate 224TMH using the Potoff model without modifying any of the parameters. We can then consider refining the Potoff parameters using MBAR-GCMC or MBAR-ITIC. Also, I have begun transferring the Potoff model into an ex-LJ potential (e.g., 11-10-9-8-7-6) which might perform better at extreme pressures.

[ramess101]

@Kaiveria @mostafa-razavi @jrelliottoh

Here are the 224TMH Saturation results:

Some important but subtle issues here:

1) The REFPROP curve is actually the TDE predicted fit, which has rather large error bars (like 10%):

2) Simulations were run with the TDE saturated liquid density (see below), which is not as reliable as REFPROP typically (could use force field density to be rigorous)

3) We are still using a 1.0 nm cut-off, since this did not significantly impact octane results, however we might want to validate this

But since the challenge is for the eta-P curve at high pressures, this was primarily just to verify that Potoff works well for this compound. We don't really need saturation.

[ramess101]

@jrelliottoh @jpotoff @msoroush @mostafa-razavi @Kaiveria

Here are some preliminary results for the challenge compound. I am still rerunning some state points to get a lower uncertainty (uncertainties are not included in figure but are typically 10%):

The interesting shape of the viscosity-pressure trend where it has an inflection point around 400 MPa is actually a heavily debated issue for elastohydrodynamic lubricants. We need to really nail down these points with low uncertainties so that we can say definitively that the Potoff S/L force field predicts this behavior.

I just wanted to make you all aware of this to see if anyone is concerned by this shape. I will provide an improved figure once we have more reliable uncertainties.