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 D4h 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 L2,3 peak is dominated by a ligand-centered excitation (Ligand (b1g) → a1u), whereas the shakeup satellite arises primarily from metal-to-ligand excitations (Metal (Cu 3d) → a1u).