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Pedro Silva
pedros@ufp.edu.pt

I am reasonably acquainted with Firefly, but I have just met a nasty problem I cannot solve:
in one of my RODFT numerical hessian computations, I get an irrealistycally high negative frequency (-55400 cm-1).

The numerical computation of the hessian involves the computation of energy of the structure upon small sequential displacements of each atom along the x, y and z axis, and these values are then used by FF to compute the hessian matrix.
I found that although most of these energies in my molecule are around -1860.68, several SCF cycles yield final results around -1860.40. this is obviously the reason for the wrong computation of frequencies, but i have not been able to make the "wrong" SCF cycles to converge into the proper energies around -1860.68. I have used very small VIBSIZ (0.001) and I keep getting trouble in only some SCF cycles, but those are enough to throw the computation to the garbage bin.
My system is MULT=2, the basis set is 6-31+G* (plus the SBKJC pseudopotential in an iron atom), I include initial guess orbitals from a previous computation, $SCF DIIS=.t. $END

Any suggestions?

Thank you!
Pedro S.


PS: I can provide the input file upon request. I did not attach it because of its 6 Mb size...