Large energy fluctuations in Al₄ TD-CI along MD trajectory

[hamidzabihi]

Hi,
I’m running hot electron relaxation MD for Al clusters using:
INDO C.I.=(12,20) CHARGE=0 RELSCF=0.0001 ALLVEC WRTCONF=0.0 WRTCI=20 TRIPLET
For Al₄, the TD-CI energies show large, seemingly unphysical fluctuations between MD steps. This does not happen for smaller
or larger clusters.
I suspect the high symmetry of Al₄ is causing near-degenerate states and numerical instability. Including TRIPLET did not resolve it.
Has anyone seen this behavior? Suggestions to stabilize the TD-CI energies along MD (e.g., CI space, symmetry breaking, or other keywords) would be greatly appreciated.
Thanks!

[godotalgorithm]

Could you provide an example of two Al4 clusters with similar geometries that have unphysically large differences in excitation energies? The tetrahedral Al4 cluster appears to have a degenerate ground state in the INDO calculation, which might be messing with how the excitation energies are being referenced (e.g. different distorted configurations might be breaking the ground-state degeneracy differently). I'm not aware of any numerical issues with INDO, so I'll need examples to understand what a problematic case even looks like.

Also, why are you using C.I.=(12,20)? The first argument is the number of active orbitals, and the second argument its the number of doubly-filled orbitals, so the 2nd argument cannot be larger than the first. According to the output file, this option is being forced to C.I.=(12,6) for the calculation to proceed.

Also, keep in mind that INDO is providing the energy difference between an electronic ground state and an electronic excited state. It is not providing the total energy of a molecule in an electronically excited state. MOPAC's similar MECI features can provide a total energy in an electronically excited state, although based on the PMx models, not the INDO model (and PMx may not be very accurate for simple metal clusters). Otherwise, you may need some other way to generate a ground state total energy to add to the electron excitation energy to form an excited total energy (e.g. an interatomic potential).