opt freq gaussian calculation is performed, I got normal termination , however, the structure has nottotaly converged. because I had no convergence in Maximum Displacement! how can I solve this problem! please help
thanks in advance.
Hi Meriem,
Try to change the geometry and clean several times, if clean does not work then only change the geometry as available for a standard structure in literature or elsewhere like pubchem, chemspider etc...
There is no particular method for this problem as per my little knowledge. Just trial and error method is recommended.
All the best,
Milan
--> Have you attempted to run the calculation with an increase in the number of iterations for the SCF cycle?
Is the SCF energy is oscillating (going up and down) or decreasing steadily alongside the iterations?
You may need to start with a better initial guess structure.
I am not sure what method your are using but start with a low level of theory with less amount of basis sets functions (HF) prior to using a higher level of theory (CCSD(T)).
What type of convergence criteria you are using (tight, superfine, exc.)?
All the previous suggestions are correct. An analysis of the reasons affecting your convergence steps would be required to give a sound reply. This said, there are a couple of hints that could fix the convergence problem.
First, you should try and increase the number of SCF interactions (scf=maxcycle=...). If your workstation is fast enough and your system not too large, you can try 1024 interactions. Otherwise, try increasing the number of interactions gradually.
Should all this fail, try a (Slow!) quadratic convergence method (scf=qc).
Hope this may help.
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