Chemistry - Convergence issue in Gaussian

Solution 1:

Is not it just the case?

Looks like it is indeed not the case. I think the phrase OP quoted from the manual, which says that the optimization stops when

The Maximum Force and RMS Force are two orders of magnitude smaller than the thresholds shown, regardless of the values of the displacements.

can be interpreted a bit differently, but I guess that algoritmically it is implemented in a pretty simple way: the above mentioned quantities should be at least 100 times smaller that the corresponding thresholds. And this is not the case for the calculation under consideration: Maximum Force value (0.000005) is not 100 times smaller (but only 90) than the threshold (0.000450).

Solution 2:

The actual answer to this question is that the Hessian is calculated analytically for DFT methods when you do a frequency calculation, but it is estimated when performing a geometry optimization. Therefore, in some cases, the optimization will show a converged structure but the frequency analysis shows that it is not below the convergence thresholds when the analytical Hessian is generated. The best option is to continue from the checkpoint file of the frequency calculation using freq opt=ReadFC guess=Read. This is all discussed on the Gaussian website here.

Oftentimes, the structure optimized from the analytical Hessian is nearly identical to the one from the approximated Hessian, but you simply don't know in advance how big this difference will be. In large, flexible molecules (ones that can have moderate displacements even at low forces), I have seen that the optimization can sometimes actually have a long way to go after continuing from the analytical Hessian.