I was drawing one structure in Gaussian 09 and optimizing it by the PM6 method. But my link died after a few minutes. I have checked the Output Text file and it said "Delta-x Convergence NOT Met".
I did not find any issues with the structure. I have tried by drawing the structure again but the problem persisted.
Can anyone help me fix this issue?
This is an usual error of IRC task (rather than optimization task). If you encountered this problem in IRC task, adding "LQA" option in IRC keyword will solve it. There are also other ways that may avoid it, such as using GS2 algorithm instead of the default HPC algorithm, decreasing stepsize, using "calcall" option and so on.
1. you can increase the number of steps
2. you can use scf=qc or xqc depending upon the choice of your system
3. you can specify the no of steps
4. you should provide more details on your input file (at least the #-part). Are you looking for a transition state or a minimum?
Maybe the structure is far from the real geometry (e.g. too short/long bonds).
Check it and, if it is a organic molecule, you could use molecular mechanics for a first (and very fast) optimization.
If the structure is not the problem, try to write scf=(maxcycle=500) (or any big number) or scf=(xqc), as Anwesh Pandey recommended, after "PM6" in your input.
And, if everything fails, as Christian Feiler said, you can provide more details on your input (including initial geometry) to try to find the problem.
Hope it helps.
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