It would be convenient to make bugfix releases on pre-2.0 branches while 2.0 is still unreleased. This is not possible without violating semantic versioning because there are breaking changes in master that appeared after 1.8.1, specifically the switch of some quantities to atomic units.

Relevant PRs need to be reverted and will instead be gradually added to main. These are:

• make optdone-as-list the default #1404
• Parse all energies as atomic units #1405
• NWChem: thermochemistry parsing cleanup #1413 ok to keep
• ADF: set unknown core MO energies to NaN #1414
• Psi4: implement thermochemistry #1415 no breaking changes, only additional implementation consistent with docs
• Jaguar: implement entropy, enthalpy, and freeenergy #1416 no breaking changes, only additional implementation consistent with docs
• Make all parsed thermochemistry consistently in atomic units #1422 bugfix for Q-Chem consistent with docs

#1412 is ok to keep.