Franck-Condon Simulation of Vibrationally-Resolved X-ray Spectra for Diatomic Systems: Validation of Harmonic Approximation and Density Functional Theory

TL;DR

This study systematically validates the use of density functional theory and the harmonic approximation in simulating vibrationally-resolved X-ray spectra of diatomic molecules, highlighting the importance of anharmonic effects and functional choice.

Contribution

It provides a comprehensive validation of DFT and harmonic approximation for diatomic X-ray spectra, and demonstrates the significance of anharmonic effects using quantum wavepacket dynamics.

Findings

DFT with specific functionals accurately predicts spectra for most diatomic molecules.

Anharmonic calculations better reproduce complex vibronic structures with significant geometrical changes.

Harmonic and anharmonic approaches yield similar results for molecules with smaller structural changes.

Abstract

Under the Franck-Condon approximation, we systematically validated the performance of density functional theory (DFT) and the effects of anharmonicity in simulating C/N/O K-edge vibrationally-resolved X-ray spectra of common diatomic molecules. To get ``transparent'' validations, vibronic fine structures of only the lowest 1s excited or ionized state in the X-ray absorption (XAS) or photoelectron (XPS) spectra were investigated. All 6 systems (N$_2$, N$_2^+$; NO, NO$^+$; CO, CO$^+$) were studied within the harmonic oscillator (HO) approximation using DFT with four functionals (BLYP, BP86, B3LYP, M06-2X) for 10 XAS and 4 XPS spectra, and excellent agreement between theoretical and experimental spectra was found in most systems, except O1s XAS of NO, CO, and NO$^+$. We analyzed and established a connection between their complex vibronic structures (many weak oscillating features within a broad peak) and the significant geometrical changes induced by the O1s hole. The three spectra were well reproduced with anharmonic (AH) calculations by using quantum wavepacket dynamics based on potential energy curves (PECs) generated by DFT methods or multiconfigurational levels, highlighting sensitivity to the anharmonic effect and the PEC quality. In other examples of XAS (CO$^+$, C1s and O1s; NO, N1s) corresponding to smaller structural changes, HO and AH approaches lead to similar fine structures, which are dominated by 0-0 and 0-1 transitions. This study highlights the use of DFT with selected functionals for such diatomic calculations due to its easy execution and generally reliable accuracy. Functional dependence in diatomic systems is generally more pronounced than in polyatomic ones. We found that BLYP, BP86, and B3LYP functionals consistently exhibited high accuracy in predicting spectral profiles, bond lengths, and vibrational frequencies, which slightly outperformed M06-2X.