My understanding is that ∆Hr is a relatively weak function of temperature. When fitting we evaluate ∆Hr(T) to be thorough in fitting, but we don't lose much accuracy when we convert to Arrhenius using ∆Hr(T=1000K)(T=1000K because that's what the trees are currently fitted at).

As we set about implementing Blowers Masel rate expressions in Cantera (Cantera/cantera#987 ) we have come across this issue again. Should the Blowers Masel expression be evaluated using ∆Hr298 or ∆Hr(T) ?

Yes, Matt's right ∆Hrxn is a pretty weak function of T, but when @12Chao converts all H abstractions (except those with negative barriers) into Blowers Masel expressions, fitted at 298K but evaluated at T, the ignition delay of n-heptane changes about 3%. So not very big, but not zero.

I guess we have a few questions. 1) what did Blowers and Masel intend? 2) what actually works best? 3) what will users expect? (we have chance to influence this, as for now we are the only users).

(Blowers' PhD thesis is at http://hdl.handle.net/2142/82466 but restricted access)

For (1) the paper (https://aiche.onlinelibrary.wiley.com/doi/abs/10.1002/aic.690461015) almost comically doesn't even have the word temperature in the whole document. In general it talks about dHrxn like it isn't temperature dependent. As far as what they're actually using they say: "In order to do so we have taken all of the hydrogen transfer reactions listed in the NIST tables Westley 1980., and looked up the heats of reaction." Which makes me think they're using dHrxn298 or dHrxn0.

One thing I particularly don't like about evaluating the BM using dHrxn(T) is that it causes programmatic branching in the rate evaluation which makes it more expensive to differentiate in the reverse-mode and much harder to generate symbolic representations for. Of course NASA polynomials have the same issue, but in an ideal world I'd like to move away from them as well.

Another problem with making Ea a function of ∆Hrxn(T) is that you cannot recover an Arrhenius expression. Ea is not just a function of the reaction. I think the ∆H should be a function of other state variables: concentration (in the case of non-ideal solvents), surface coverage (for co-adsorbates in heterogeneous catalysis), etc. so that these effects that we have learned to predict for thermochemistry are also altering kinetics. Which might also make automatic differentiation tricky. But I think I'm veering towards saying it shouldn't be a function of T. (hmm. but what about temperature-dependent solvation? or crossing into a supercritical regime? there are all sorts of counterexamples and problems)

But in the context of Cantera, for example, where you have a manager keeping track of species enthalpies, that is already at the state Temperature and it's in easier to query the current ∆Hrxn(T) than a reference ∆Hrxn.

So I think we might want to think about how we format things this way:

RMG est format => RMG output => Simulator format

We use RMG to generate the rate/thermochemistry estimates in some programmatic format, we write the RMG mechanism to a file in some format and when the simulator reads the file it compiles it into a third format.

In the first case we're generating the estimates so the format is whatever our estimators spit out and we really only care that the format allows the estimators to perform well. In the simulator format we care that we've represented the estimation format properly, but we also care a lot about our ability to evaluate it optimally. The RMG output can be anything as long as it has enough information to generate simulator format.

Following the solvation example. We could have RMG output that is solvent specific. We could also output something much more similar to the RMG est format and let the simulator interpret that to whatever format it thinks is most convenient. These put different burdens on different software, but I think neither of these prevents the simulator from compiling its own kinetics format at the end.

@sevyharris and I just spent a while getting very confused as to why our Blowers-Masel fit was not fitting the training reaction when evaluated with H(T) but only with H(1000K). Turns out it's because we had a single training reaction, and this line

Having been thinking about this quite a bit (in the context of adding BM to Cantera) I'm leaning towards the conclusion that Blowers and Masel intended that it should be H(298K) not H(T). In any case, I think we should be consistent!

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