Everson da Silva Gomes Whenever feasible, a frequency calculation should be performed after a geometry optimization (OPT). This step is essential because it verifies whether the optimized structure corresponds to a true minimum on the potential energy surface—indicated by the absence of imaginary frequencies—or a saddle point, which would exhibit one or more negative frequencies.

It is also important to note that the optimization algorithms used in Gaussian and most quantum chemistry software are designed to locate local minima rather than the global minimum. As a result, the final optimized structure may represent a metastable conformation rather than the most stable one. To increase confidence in obtaining the true minimum, additional strategies are often necessary. These include manually adjusting initial geometries (such as rotating torsional angles), performing systematic conformational scans, or employing advanced techniques like metadynamics or global optimization algorithms when available. Furthermore, the results should always be interpreted in the context of chemical intuition and, whenever possible, validated against higher-level theoretical methods or experimental data.

In this context, the results you are observing are completely normal. Geometry optimizations in quantum chemistry typically converge to the nearest local minimum, which may not necessarily be the global minimum or the most chemically relevant structure. The goal is to either locate the global minimum or identify a structure that aligns with established chemical principles and available experimental data.

Good luck!

Lucas Gian Fachini Thank you very much (Obrigado!)