Terahertz spectral analysis of nicotinamide: A comparative study of density functional theory and first-principles molecular dynamics.

Density functional theory (DFT) is widely used in the terahertz spectral analysis of organic molecules; however, its inherent 0 K harmonic approximation cannot capture atomic thermal fluctuations and lattice anharmonicity, leading to significant discrepancies between theoretical and experimental spectra. In this study, taking nicotinamide crystals as a model system, we systematically compare the spectral prediction capabilities of conventional DFT and temperature-inclusive first-principles molecular dynamics (FPMD) in the 0.4-2.0 THz range under identical computational settings. Experimental results show that, due to the incorporation of anharmonicity and thermal motion, FPMD achieves markedly better agreement with experimental peak positions than DFT, reducing the maximum deviation from 165% to 12.6%. Furthermore, by analyzing the FPMD trajectories with the averaged modified independent gradient model (amIGM), we reveal from a dynamic perspective the weak intermolecular interactions in nicotinamide, where van der Waals interactions dominate and hydrogen bonds are predominantly confined to the vicinity of functional groups, clarifying how temperature modulates these interactions and consequently affects the spectra. This study provides an integrated framework combining dynamic modeling and weak interaction analysis for the theoretical prediction of terahertz spectra of organic crystals, and offers a methodological reference for investigating the formation mechanisms of characteristic terahertz absorption peaks.