The time needed to reach the convergence depends on the number of cycles you need not the maximum number set. 20 cycles are often not sufficient. Though it's worth checking the course of the optimisation from time to time. It's possible your initial structure is far from the transitionstate. Even to far to convergence to the right structure. Check the changes of the structure visually.

In addition have a look at the changes of energy. A small gradient can result in slow convergence. In this case change the parameters for damping.

Mr. Mondal,

The answer to your question depends on: (i) The theoretical method; and (ii) 3D molecular and electronic structures of the molecule.

For example, if you use BOMD or ADMP methods to study TS, then a frequency analysis at each trajectory step provides a direct information about PES. The optimization can be carried out at about 50 cycles.

Koushik Mondal

Although the default value for cycles are 20, but as such there is not optimum cycles to reach to a convergence. When the structure is very far from the optimum, it may take long way to reach to minima using large numer of cycles. Hence, it is advisable to keep an eye on your submitted job time to time how it converges by checking the convergence parameters in your log file.

You can use the following command to check progress of your convergence:

grep Done name.log

It will show the SCF done. Moreover, you can check the convergence parameters in your log file where you can get a table for Max. force, Max. RMS, displacement with converfence indicator.

Thanks Shashi Kumar for the clarification.