Mismatched conformer degrees of freedom pass silently through TST, then surface as a bare "Singular matrix" in the MSC solve

Ran into both of these while splicing QM transition states into an otherwise RMG-estimated pressure-dependent network, and they compound in a way that made the diagnosis much harder than it needed to be.

If the transition state conformer carries translation and rotation but the wells are vibration-only (or the other way round), Reaction.calculate_tst_rate_coefficient (rmgpy/reaction.py:1441) just multiplies the partition functions together — nothing anywhere in it looks at what modes either side actually has. In my case Q‡/Q_reactant was off by ~1e14 and k_TST(3200 K) came out at 1.8e26 s^-1 for a unimolecular step.

That rescales k(E), and the modified strong collision solve then dies at np.linalg.solve(a_mat, b) in rmgpy/pdep/msc.pyx with nothing but numpy.linalg.LinAlgError: Singular matrix — no grain, no J, no temperature, no pressure. I ended up bisecting by hand down to grain 346 / J=0 / 3200 K / 0.1 bar.

The bit that struck me is that the intent is already there a few lines up: b gets checked with np.isnan and logs a warning naming the grain and the isomer. a_mat has no equivalent check, and it's the argument that actually fails.

A warning in calculate_tst_rate_coefficient when the reactant and TS conformers carry different mode classes, or just the grain/J/T/P coordinates on the singular-matrix failure, would have turned a two-hour hunt into an immediate error message. Either one alone would have done it.

One thing worth flagging for anyone fixing this: the loud failure only happens when the mismatch is asymmetric. I checked afterwards, and if you put translation and rotation on the wells and the TS alike, the partition functions cancel again — the solve succeeds and the rate coefficient comes back unchanged, on a network that's still wrong. So the singular matrix isn't a general tripwire for this, which is an argument for the warning rather than relying on the crash.

Happy to open a PR for both.

Start with Reaction.calculate_tst_rate_coefficient in rmgpy/reaction.py:1441 and the np.linalg.solve call in rmgpy/pdep/msc.pyx. Trace the existing b NaN warning and determine whether mismatched mode classes or singular-matrix context should be reported. Done means the asymmetric mismatch or solve failure reports actionable diagnostic details instead of failing silently or with only "Singular matrix".