Principal NTOs for the S 0 → S 1 and S 0 → S 3 excitations of this dipeptide are depicted in Fig. . The S 1 and S 2 states both have nπ * character, involving different carbonyl groups, but are otherwise qualitatively similar to one another so only one of them is considered. The S 0 → S 3 transition involves a Rydberg state. Each of these transitions is well-described by a single eigenvalue of ∆P elec , with λ 2 1 > 0.94. For n > 2 sets of diffuse functions, the excitation energies change by ≤ 0.001 eV (Table ) and additional diffuse functions also have vanishingly little impact on the nature of the NTOs, as shown in the side-by-side comparison of 6-31++G* and 6-31(9+,9+)G* results that appears in Fig. . However, several of the CT metrics continue to change their values as additional diffuse basis functions are added.