Accurate N1s and C1s XPS spectra of g-C3N4 were obtained by a combined cluster-periodic approach and we make new assignments.

Truncated cluster models represent an effective way for simulating X-ray spectra of 2D materials. Here we systematically assessed the influence of two key parameters, the cluster shape (honeycomb, rectangle, or parallelogram) and size, in X-ray photoelectron (XPS) and absorption (XAS) spectra simulations of three 2D materials at five K-edges (graphene, C 1s; C3N, C/N 1s; h-BN, B/N 1s) to pursue the accuracy limit of binding energy (BE) and spectral profile predictions. Several recent XPS experiments reported BEs with differences spanning 0.3, 1.5, 0.7, 0.3, and 0.3 eV, respectively. Our calculations favor the honeycomb model for stable accuracy and fast size convergence, and a honeycomb with ~10 nm side length (120 atoms) is enough to predict accurate 1s BEs for all 2D sheets. Compared to all these experiments, predicted BEs show absolute deviations as follows: 0.4-0.7, 0.0-1.0, 0.4-1.1, 0.6-0.9, and 0.1-0.4 eV. A mean absolute deviation of 0.3 eV was achieved if we compare only to the closest experiment. We found that the sensitivity of computed BEs to different model shapes depends on systems: graphene, sensitive; C3N, weak; h-BN, very weak. This can be attributed to their more or less delocalized π electrons in this series. For this reason, a larger cluster size is required for graphene than the other two to reproduce fine structures in XAS. The general profile of XAS shows weak dependence to model shape. Our calculations provide optimal parameters and accuracy estimations that are useful for X-ray spectral simulations of general graphene-like 2D materials.

Cu-O$_2$ structures play important roles in bioinorganic chemistry and enzyme catalysis, where the bonding between Cu and O$_2$ parts serves as a fundamental research concern. Here we performed a multiconfigurational study on the copper L$_{2,3}$-edge X-ray absorption spectra (XAS) of two copper enzyme model complexes to gain a better understanding of the antibonding nature from clearly interpreted structure-spectroscopy relation. We obtained spectra in good agreement with the experiments by using the restricted active-space second-order perturbation theory (RASPT2) method, which facilitated reliable chemical analysis. Spectral feature interpretations were supported by computing the spin-orbit natural transition orbitals. All major features were assigned to be mainly from Cu $2p$ to antibonding orbitals between Cu $3d$ and O$_2$ $\pi^{*}$, Cu$3d-\pi^*_\text{O-O}$ (Type A), and a few also to mixed antibonding/bonding orbitals between Cu $3d$ and O$_2$ $\pi$, Cu$3d\pm\pi_\text{O-O}$ (Type M). Our calculations provided a clear illustration of the interactions between Cu $3d$ and O$_2$ $\pi^{*}$/$\pi$ orbitals that are carried in the metal L-edge XAS.

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