Evaluating the Performance of the Exact Integral Simplified Time-Dependent Density Functional Theory (XsTD-DFT) to Compute One- and Two-Photon Absorption

Computing one-photon absorption and two-photon absorption (1PA and 2PA) of large molecular systems using an all-atom quantum mechanical (AQM) methodology presents significant computational challenges. This study evaluates the performance of the exact integral simplified time-dependent density functional theory (XsTD-DFT) method, a new recently introduced simplified quantum chemistry (sQC) approach that can be used as a key ingredient for AQM workflows to compute 1PA and 2PA. The reliability, robustness, and computational efficiency of the XsTD-DFT scheme are assessed against RI-CC2 reference calculations as well as with respect to TD-DFT results for a set of 91 organic molecules that includes systems from the QUEST database, medium-sized push–pull molecules, and small microhydrated clusters. The impact of various exchange–correlation functionals, basis sets, and the single energy threshold used to truncate the CIS space in the XsTD-DFT procedure is investigated. Results show that the XsTD-DFT method provides a substantial computational speed-up compared to TD-DFT, up to 3 orders of magnitude in CPU time with an energy threshold of 9 eV. The method robustly reproduces TD-DFT trends for excitation energies, oscillator strengths, and 2PA strengths. XsTD-DFT demonstrates reliability in capturing structure–property relationships and is reliable for nonequilibrium geometries as well as microhydrated systems. Comparisons with respect to experimental 1PA and 2PA spectra recorded in solution show similar agreements for the XsTD-DFT scheme than for TD-DFT. Guidelines for using the XsTD-DFT method in the context of 1PA and 2PA are provided. This study concludes that XsTD-DFT is a reliable, robust, and computationally efficient method to compute 1PA and 2PA, making it well-suited as a key ingredient for AQM workflows to treat realistic systems.