Include BACs to obtain accurate k_rev

[kspieks]

Is your feature request related to a problem? Please describe. When using ARC to run Arkane to obtain a rate constant, ARC currently has bac_type=None for all kinetics calculations so k_fwd is meaningful but the species thermo, Keq, and k_rev are not. This works well if the user wants to create a library of k_fwd before an RMG job. However, it may be useful to add an optional argument to indicate the direction and to include BACs; these would allow the other quantities present in rxns/ts/arkane/output.py to be meaningful.

Describe the solution you'd like The default behavior should be preserved. Some optional features to consider are

• call Arkane once without BACs and again (still without BACs) to generate another kinetic library of all the reverse rates. This would likely require the user to decide which library should be used and would only be helpful for small batches
• call Arkane once without BACs to obtain an accurate k_fwd and again with BACs to obtain an accurate Keq that could be used to obtain an accurate k_rev. Something to keep in mind is that the reaction should be written in the direction that has the reactant and TS as most similar, which gives better cancellation of errors in the calculation of k_fwd. Typically, this means writing in the exothermic direction, but there are exceptions in which the endothermic direction better supports this goal, such as bond scission in a ring since the TS will be cyclic like the reactant

[alongd]

As we discussed offline, it could be beneficial to compute the rate in a certain direction by default, as you mentioned, regardless of BACs.