The problem is that [1] the the two input geometries are not aligned and [2] sequence of atoms are different (e.g., in structure 1, 25th atom is Fe, whereas in structure 2, 25th atom is O). Atom labels should be exactly the same in all the guesses and there should be minimum deviation/geometries should be aligned.

Hi Julián Sánchez

Link 101 reads title and molecular specification. You should delete "geom=connectivity" in route section. This keyword usually do not required.

Best regards

The problem is that [1] the the two input geometries are not aligned and [2] sequence of atoms are different (e.g., in structure 1, 25th atom is Fe, whereas in structure 2, 25th atom is O). Atom labels should be exactly the same in all the guesses and there should be minimum deviation/geometries should be aligned.

Mr. Sánchez,

Try transition state analysis via BOMD or ADMP.

A lot of examples are shown here:

https://novapublishers.com/shop/protonation-properties-applications-and-effects/

Uttam nailed the issue. The atoms should be exactly in the same order in both input structures provided to QST2.

Uttam is right, also the set of keywords does not really looks healthy.

estmfc geom=connectivity temperature=343 scf=fermi

I would remove that all during initial search of transition state.