Assessing Nonperiodic and Periodic TD-DFT: The Case of Photochromism in Aluminosilicates

Excited-state simulations of materials involving localized transitions on dopants or chromophores are typically performed within an “embedded-cluster” approach, where the photoactive moiety is embedded in its environment according to various approximations. In recent years, fully periodic time-dependent density functional theory (TD-DFT) implementations with an acceptable computational cost have emerged. Periodic TD-DFT provides an attractive alternative to the tedious embedding approaches but has not been extensively tested. Here, we investigate photochromism as a case study for which experimental benchmark data is available. We compare the electrostatic embedding approach to periodic TD-DFT for 14 different aluminosilicates, 9 of which are characterized experimentally. The photochromism in these materials invokes three types of electronic transitions: localized (Fcenter), charge-transfer, and triplet-to-singlet transitions, making the system particularly rich for assessing the general usefulness of various methods. We start by discussing the computational approximations and their influence on the results for both the periodic and the embedded-cluster approaches. Overall, we find that the periodic TD-DFT calculations are computationally affordable, require less user-dependent choices (e.g., size of the cluster and the nature of the embedding), and generally yield results in good agreement with the experiment. Still, the cluster computations have the unique advantage of enabling levels of theory beyond TD-DFT (here we performed CIS(D)), which are still too costly in the fully periodic approach. For the cases studied herein, the embedded-cluster approach, including when using CIS(D), is, however, prone to artifacts, and the CIS(D) values agree rather less than more with the experiment compared to TD-DFT, despite their significantly higher computational cost. Therefore, we encourage further investigations into the transferability of the accuracy reached by periodic TD-DFT and the careful analysis of the corresponding results (here exemplified by the Λ index, measuring the localized character of the excitation) to gain more insights into excited states in the condensed phase.