I had a same problem for C60. But I used "freq numerical" and worked fine for me.
If you have symmetry (I think at least can be D5h for your case) you have just 5 unique atoms. (I think)
I think you can get very accurate result (nearly same as analytical) if you have accurate integration.
I got accurate and correct results for C60 just by setting this criteria
CONVERGENCE energy 1e-8 \
density 1e-6 \ ## maybe not necessary
gradient 1e-5 ##maybe not necessary
GRID fine
TOLERANCES tight
Also I optimized with the tight criteria.
Results were in excellent agreement with experiment.
and on 80 processor took less than 5 hours. (not too much)
I hope it help.
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