Deterministic test needed for short-FeCO6

[prckent]

Is your feature request related to a problem? Please describe.
Given #3820, this is a straightforward way to avoid repeats and also adds a much less trivial molecular case to our test set that is run in CI.

Describe the solution you'd like
Add deterministic tests for FeCO6

Describe alternatives you've considered

Test another non-trivial molecule - but we have the files for FeCO6 already. H2O dimer also exists but has fewer species.

Additional context