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Alex Nest
comconadin@gmail.com

We have very strange problem with MCSCF geometry optimization of Fe3S4 cluster (it is kind of protein Fe-S clusters). We have done the following things:

1) first we know (from refs) that our cluster with charge -2 has multiplicity 2.
2) we selected the basis 6-31G** (non-Fe) / LANL2DZ (Fe) and prepare input for C3 symmetry
3) we tried to perform ROHF, but we could not converge it. Only UHF converges but very hard and with very high value of S-SQUARED (~ 6.0 that indicates very large spin contamination). We decided that ground state could be multi-reference and decided to perform MCSCF.
4) we remove one electron and converge RHF for such system
5) then we look at orbitals and selected (3,3) active space
6) then we take vector from (4) step and start MCSCF convergency (it was not hard).
7) then we started MCSCF geometry optimization and faces the following problem:
rms grew:
         MAXIMUM GRADIENT =  0.2556168    RMS GRADIENT = 0.0464457
         MAXIMUM GRADIENT =  0.2841621    RMS GRADIENT = 0.0537972
         MAXIMUM GRADIENT =  0.4198081    RMS GRADIENT = 0.0725880
         MAXIMUM GRADIENT =  0.5151014    RMS GRADIENT = 0.0871977
         MAXIMUM GRADIENT =  0.5138660    RMS GRADIENT = 0.0859293
         MAXIMUM GRADIENT =  0.5365677    RMS GRADIENT = 0.0896454
         MAXIMUM GRADIENT =  0.5712724    RMS GRADIENT = 0.0946583
         MAXIMUM GRADIENT =  0.6041863    RMS GRADIENT = 0.1003238
         MAXIMUM GRADIENT =  0.6494662    RMS GRADIENT = 0.1071054
         MAXIMUM GRADIENT =  0.6630896    RMS GRADIENT = 0.1096243
         MAXIMUM GRADIENT =  0.6674327    RMS GRADIENT = 0.1108875
         MAXIMUM GRADIENT =  0.6796559    RMS GRADIENT = 0.1108236
         MAXIMUM GRADIENT =  0.7419342    RMS GRADIENT = 0.1189649
         MAXIMUM GRADIENT =  0.7598804    RMS GRADIENT = 0.1230276
         MAXIMUM GRADIENT =  0.7938448    RMS GRADIENT = 0.1285953
         MAXIMUM GRADIENT =  0.8451196    RMS GRADIENT = 0.1364123
         MAXIMUM GRADIENT =  0.8660177    RMS GRADIENT = 0.1402934
         MAXIMUM GRADIENT =  0.8763449    RMS GRADIENT = 0.1411417
         MAXIMUM GRADIENT =  0.9124437    RMS GRADIENT = 0.1468867
         MAXIMUM GRADIENT =  0.9405698    RMS GRADIENT = 0.1504783
         MAXIMUM GRADIENT =  0.9340931    RMS GRADIENT = 0.1504645
         MAXIMUM GRADIENT =  1.0102995    RMS GRADIENT = 0.1625614
but energy decreased:
         NSERCH=  0     ENERGY=   -3269.0048828
         NSERCH=  1     ENERGY=   -3269.0154026
         NSERCH=  2     ENERGY=   -3269.0019545
         NSERCH=  3     ENERGY=   -3268.9884073
         NSERCH=  4     ENERGY=   -3268.9871148
         NSERCH=  5     ENERGY=   -3268.9903808
         NSERCH=  6     ENERGY=   -3268.9930019
         NSERCH=  7     ENERGY=   -3268.9968767
         NSERCH=  8     ENERGY=   -3268.9989956
         NSERCH=  9     ENERGY=   -3269.0010380
         NSERCH= 10     ENERGY=   -3269.0028638
         NSERCH= 11     ENERGY=   -3269.0016070
         NSERCH= 12     ENERGY=   -3269.0037472
         NSERCH= 13     ENERGY=   -3269.0063914
         NSERCH= 14     ENERGY=   -3269.0094045
         NSERCH= 15     ENERGY=   -3269.0083837
         NSERCH= 16     ENERGY=   -3269.0083270
         NSERCH= 17     ENERGY=   -3269.0083347
         NSERCH= 18     ENERGY=   -3269.0083693
         NSERCH= 19     ENERGY=   -3269.0064975
8) we tried SA-MCSCF (adding wstate=1,0.1 to $DET and ntrack=2 to $MCSCF) but approximately the same picture was obtained.

Firefly v.8.1.1 was used. Can you advice something in our case? Could you tell if our modelling strategy is right in global?

Attached please see the input of step (7).

This message contains the 1403 kb attachment [ casopt.inp ] MCSCF optimization from step (7)