When you ask such question, please provide details on the methods you are using.

General remarks: increase scf cycles to allow for convergence and start with a better guess/initial geometry, first pre-optimize at a low level of theory

Possible problems could be improper QM level, unfavorable configuration, wrongly assigned point group and so on.

What method and basis set are you using? I would also suggest to start with a better initial guess for the geometry and if possible optimize beforehand with a lesser level of theory something that may take less than half a day to complete. I would also advise to increase the amount of SCF cycles, or maybe you need a tighter convergence criteria.

Thank you all. The problem is solved by removing the keyword "scfconf=7".

Hi Sasthi,

Although you may achieve convergence with lower scfconv criterion, this is not a good choice. The wavefunction could be unstable and often this lowered convergence criterion is not recommended, especially When you deal with difficult systems. The outcome could be non-continuous energy profile and broken configurations in successive MD steps.

Dear Zhaoxi,

Thank you for your suggestion. I have got the optimized structure but its little bit distorted as you have said. So, what will be the possible way to overcome this problem? Please suggest me something.

Thanking you

Sasthi