Simplified quantum chemistry methods to evaluate non‐linear optical properties of large systems

This review presents the theoretical background concerning simplified quantum chemistry (sQC) methods to compute non‐linear optical (NLO) properties and their applications to large systems. To evaluate any NLO responses such as hyperpolarizabilities or two‐photon absorption (2PA), one should evidently perform first a ground state calculation and compute its response. Because of this, methods used to compute ground states of large systems are outlined, especially the xTB (extended tight‐binding) scheme. An overview on approaches to compute excited state and response properties is given, emphasizing the simplified time‐dependent density functional theory (sTD‐DFT). The formalism of the eXact integral sTD‐DFT (XsTD‐DFT) method is also introduced. For the first hyperpolarizability, 2PA, excited state absorption, and second hyperpolarizability, a brief historical review is given on early‐stage semi‐empirical method applications to systems that were considered large at the time. Then, we showcase recent applications with sQC methods, especially the sTD‐DFT scheme to large challenging systems such as fluorescent proteins or fluorescent organic nanoparticles as well as dynamic structural effects on flexible tryptophan‐rich peptides and gramicidin A. Thanks to the sTD‐DFT‐xTB scheme, all‐atom quantum chemistry methodologies are now possible for the computation of the first hyperpolarizability and 2PA of systems up to 5000 atoms. This review concludes by summing‐up current and future method developments in the sQC framework as well as forthcoming applications on large systems. This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Structure and Mechanism > Molecular Structures Electronic Structure Theory > Density Functional Theory Electronic Structure Theory > Semiempirical Electronic Structure Methods Simplified quantum chemistry methods provide a new route for the all‐atom computation of nonlinear optical response properties of large systems.