Discuss deprecation of chemically-activated reaction type

[ischoegl]

With the introduction of YAML, there's an opportunity to discontinue the separation of FalloffReaction and ChemicallyActivatedReaction prior to release to production. The classes differ in a single detail, i.e.

cantera/src/kinetics/GasKinetics.cpp

Lines 144 to 148 in 719cb3d

	if (reactionType(m_fallindx[i]) == FALLOFF_RXN) {
	pr[i] *= m_rfn_high[i];
	} else { // CHEMACT_RXN
	pr[i] *= m_rfn_low[i];
	}

Rather than specifying a chemically activated reaction as a separate type:

- equation: ch3 + oh (+M) <=> ch2o + h2 (+M)  # Reaction 1
  type: chemically-activated
  low-P-rate-constant: {A: 282320.078, b: 1.46878, Ea: -3270.56495}
  high-P-rate-constant: {A: 5.88e-14, b: 6.721, Ea: -3022.227}
  Troe: {A: 1.671, T3: 434.782, T1: 2934.21, T2: 3919.0}

the following equivalent would be sufficient:

- equation: ch3 + oh (+M) <=> ch2o + h2 (+M)  # Reaction 1
  type: falloff
  chemically-activated: true
  low-P-rate-constant: {A: 282320.078, b: 1.46878, Ea: -3270.56495}
  high-P-rate-constant: {A: 5.88e-14, b: 6.721, Ea: -3022.227}
  Troe: {A: 1.671, T3: 434.782, T1: 2934.21, T2: 3919.0}

i.e. over-riding chemically-activated being false by default for falloff reactions.

It's also relatively straight-forward to deprecate the remaining instantiation calls (i.e. XML and from scratch). Changes in the converters (cti2yaml/ctml2yaml) are likely trivial.

[speth]

I think the key question is whether a "chemically activated" reaction is a falloff reaction or not. I'm not sure, and I'd appreciate some input from the chemists on this point: Is "chemically activated falloff reaction" a reasonable description for this type of reaction?

If this is the case, we could combine the two YAML reaction types. However, I would suggest making no changes to the XML / CTI formats, and waiting until those formats are deprecated to deprecate the corresponding C++ class.

[rwest]

I find it helpful to re-read section 3.6 of the Chemkin Theory Manual about once a year, and yet I'm still not totally sure. They're definitely closely linked. But is it any fairer to say that chemically-activated is a type of fall-off than it is to say fall-off is a type of chemically-activated? How do you choose which is the default?

[ischoegl]

@rwest ... thanks for the link! Comparing Figs. 3-1 and 3-3 there is an expected left-right flip. For the specific example given, both go through a chemically activated compound (C2H6*), and both have a fall-off behavior (either to the left or right). The real distinction appears to be whether the reaction type is 'unimolecular/recombination' or 'bimolecular'.

I.e. a bimolecular (or chemically-activated) designation is not even necessary (!), as the type of reaction is already specified by the reaction equation itself, i.e. simply specifying falloff as type would be sufficient for both cases. I.e.

- equation: ch3 + oh (+M) <=> ch2o + h2 (+M)  # Reaction 1: bimolecular!
  type: falloff
  low-P-rate-constant: {A: 282320.078, b: 1.46878, Ea: -3270.56495}
  high-P-rate-constant: {A: 5.88e-14, b: 6.721, Ea: -3022.227}
  Troe: {A: 1.671, T3: 434.782, T1: 2934.21, T2: 3919.0}

[speth]

That's an interesting observation, @ischoegl. If nothing else, that could at least be used as part of the validation for these reactions.

It seems to be the case that the terminology used consistently applies the term "falloff" exclusively to the unimolecular/recombination case, and "chemically activated" or "chemical activation" exclusively to the bimolecular case. Is there a name that encompasses both of these reaction types? They could both be called pressure-dependent reactions, but I don't really like that as that confuses things with the pressure-dependent-Arrhenius and Chebyshev types.

[rwest]

I think choosing the rate expression based entirely on the number of products is a dangerous precedent. There are so many cases where (sometimes for good reason) people model things are not physically based, like irreversible reactions, fractional stoichiometries, merged non-elementary reactions, etc..

[ischoegl]

@rwest .... I was going strictly by the rationale provided by CHEMKIN (which likely is a good default behavior). There is still the option to provide a flag to over-ride for non-physically motivated cases, which adds to @speth's question about appropriate nomenclature.

[ischoegl]

Closing (together with #745).