Perovskite nanocrystals (NCs) are among the most fashionable names nowadays in the field of colloidal synthesis owing to their superior photoluminescence quantum yield and blinking-free properties, which make them promising materials for both classical [1,2] and quantum light sources [3,4]. Their brightness and sub-nanosecond radiative decay originates from the inherent correlation effects [5,6]. The manybody Coulomb interaction has generally been studied for semiconductor quantum dots [7,8]. Perovskites, as a consequence of the unique properties of their dielectric functions, possess enhanced Coulomb interaction between the charge carriers [9]. This leads to large binding energies of multi-exciton systems such as trion and biexciton in these NCs or the sizable splitting in the fine structure of single exciton states. Considering each NC as an artificial atom under the envelope function approximation, this problem of correlation effects can be approached at first by using second-order many-body techniques as outlined in Ref. [10]. This offers an elegant and efficient method that provides qualitative results for the trion and biexciton binding energies [11]. In going beyond the second-order description, configuration interaction can be employed to include the correlation energies between the various charge carriers in a more holistic manner. REFERENCES: [1] Kovalenko et al., Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Science 358, 6364, 745-750, 2017. [2] Raino et al., Single Cesium Lead Halide Perovskite Nanocrystals at Low Temperature: Fast Single- Photon Emission, Reduced Blinking, and Exciton Fine Structure. ACS Nano 10, 2, 2485–2490, 2016. [3] Utzat et al., Coherent single-photon emission from colloidal lead halide perovskite quantum dots. Science, 363(6431):1068–1072, 2019. [4] Tamarat et al., The dark exciton ground state promotes photon-pair emission in individual perovskite nanocrystals. Nature Communications. 11, 6001, 2020. [5] Becker et al., Bright triplet excitons in caesium lead halide perovskites. Nature, 553(1):189–193, 2018. [6] Tamarat et al., The ground exciton state of formamidinium lead bromide perovskite nanocrystals is a singlet dark state. Nature Materials 18, 717–724, 2019. [7] M. Combescot and R. Combescot, Optical stark effect of the exciton: Biexcitonic origin of the shift. Phys. Rev. B, 40:3788–3801, 1989. [8] Rontani et al., Coulomb correlation effects in semiconductor quantum dots: The role of dimensionality. Phys. Rev. B 59, 10165, 1999. [9] Even et al., Analysis of Multivalley and Multibandgap Absorption and Enhancement of Free Carriers Related to Exciton Screening in Hybrid Perovskites. J. Phys. Chem. C, 118, 11566−11572, 2014. [10] I. Lindgren and J. Morrison, Atomic Many-Body Theory. Springer-Verlag, 1982. [11] Nguyen et al., Calculation of the biexciton shift in nanocrystals of inorganic perovskites. Phys. Rev. B, 101. 125424, 2020.