Optimization of Theoretical Models and Impact of Basis Sets on Infrared Intensities of the trans-1,2-C2H2F2 Molecule

This work addreses the analysis of fundamental infrared intensities and dipole moment derivatives of the trans-1,2-C2H2F2 molecule. The study aims to understand discrepancies between theoretical and experimental results by exploring the impact of basis sets and electronic correlation levels. Small basis sets, such as 6-31G, showed greater accuracy for high-intensity vibrational modes, such as CF stretching, due to their ability to capture simple geometric changes. Theoretical models based on DFT (B3LYP and M06L) and ab initio methods (MP2 and CCSD) were compared using metrics such as mean absolute deviation (MAD) and root mean square error (RMS). Results indicated that smaller basis sets are more suitable for describing normal stretching modes, minimizing errors and artificial effects introduced by excessive polarization functions. Additionally, the NC-NCF-O model based on MP2/6-31G(d,p) proved to be robust for determining dipole moment derivatives, yielding the lowest MAD and RMS. The study also analyzed the F2CS molecule, confirming the effectiveness of small basis sets for high-intensity vibrational modes. In conclusion, the choice of basis set is critical for accurate modeling of infrared intensities, and smaller basis sets often outperform larger ones, particularly for well-defined and localized vibrational modes.