Excited-state Properties Beyond the Excitation Energy from Orbital-Optimized Density Functional Calculations I: Dipole Moments of Rydberg States
Rydberg excited states are challenging to describe due to their highly diffuse character. Orbital-optimized density functional calculations typically provide more accurate values of the excitation energy of Rydberg states than time-dependent density functional theory approaches. However, the reliability of orbital-optimized methods for properties of Rydberg excited states such as the dipole moment remains much less explored, with existing benchmarks largely limited to the lowest excited states. Here, orbital-optimized density functional calculations with a plane-wave basis set are used to compute the dipole moment of several Rydberg states of a set of small molecules. Plane waves provide a flexible representation of diffuse Rydberg orbitals, overcoming limitations of commonly used atomic orbitals basis sets. Due to overconfinement of the Rydberg orbitals, a single-augmented atomic basis set yields a magnitude of the dipole moment that disagrees with the plane-wave calculations, even when the corresponding excitation energy is in good agreement. For the most diffuse states, the orientation of the dipole moment predicted by the atomic orbitals basis set can also be incorrect, and discrepancies with plane waves calculations persist even when extra augmented diffuse functions are added. The generalized gradient approximation functional PBE used in combination with the plane-wave representation of the orbitals gives good agreement with higher-level coupled-cluster calculations performed with sufficiently diffuse basis sets, when the latter are available. The hybrid functional PBE0 further improves the results, while PBE with globally scaled explicit Perdew-Zunger self-interaction correction generally leads to larger errors and an overestimation of the dipole moment, despite restoring the correct asymptotic $-1/r$ dependence of the effective Kohn--Sham potential.