The Cu L-edge XPS Spectrum of Copper Phthalocyanine: Quantitative Ab Initio Analysis Including Spin–Orbit Coupling and Löwdin Nonorthogonal Determinant Overlaps

We present a detailed ab initio investigation of the Cu L-edge X-ray photoelectron spectroscopy (XPS) spectrum in D 4 h copper phthalocyanine (CuPc). Spin–orbit coupling is treated with the Breit–Pauli Hamiltonian, and transition intensities are computed using the Löwdin formalism within the sudden approximation, accounting for nonorthogonality between initial and final-state wave functions. The nonorthogonal Löwdin determinant approach accurately reproduces experimental intensities, while orbital analysis highlights the charge-transfer and orbital-reorganization processes underlying the excitations. These results provide a quantitative and physically transparent interpretation of the Cu L-edge XPS spectrum, clarifying the distinct ligand and metal-centered character of the main and satellite excitations. The main L 2,3 peak is dominated by a ligand-centered excitation (Ligand (b 1 g ) → a 1 u ), whereas the shakeup satellite arises primarily from metal-to-ligand excitations (Metal (Cu 3d) → a 1 u ).