While the recent literature has focused on applying high-level ab initio and DFT methods to study INVEST compounds, we believe that some conceptual clarity has been lost in the process. Instead, we apply the simplest possible electronic structure method that can capture the physics of the problem. The Pariser-Parr-Pople (PPP) method is an extension of the semi-empirical Hückel molecular orbital theory for 𝜋-electron systems that includes electron correlation. It originated in the 1950s, with new parametrizations being developed mainly in the 1960s and 1970s, and was used in the dye industry at least until the 1980s before the advent of more accurate ab initio methods. The PPP method retains the conceptual clarity of Hückel theory, while at the same time including the electron correlation that is necessary to capture the inverted gap. For another recent application of PPP to INVEST compounds, see the work by Painelli and co-workers. Pariser-Parr-Pople theory Within Pople's formulation of PPP theory, Roothan's equations are solved self-consistently. . Under the Zero Differential Overlap (ZDO) approximation, overlap integrals are neglected for orbitals on different centers, leading to a simplification of the Roothan equations as S becomes the unit matrix