They refer to the ground-state KS orbitals Φ iσ (r), the solutions to the Coupled-Perturbed Kohn-Sham (CPKS) equations Φ Z iσ (r), and the response orbitals Φ X iσ (r) stemming from the TDA eigenvalue problem. Their corresponding basis-set expansion coefficients are denoted C µiσ , Z µiσ , and X µiσ , respectively. S represents the atomic overlap matrix,