The four parameters are thus 𝑈 𝑟 , 𝛾 𝑟𝑟 , 𝛾 𝑟𝑠 and 𝛽 𝑟𝑠 . They are derived either from experiment, first principles, or a combination of both. The Pariser-Parr approximation leads to 𝑈 𝑟 = -IP 𝑣 (6) where IP 𝑣 is the valence-state ionization potential of the orbital and atom in questions (e.g. a 2p orbital of an sp 2 -hybridized C atom). The IP 𝑣 values (and the corresponding electron affinities EA 𝑣 ) are tabulated by Hinze and Jaffé as determined from experimental data. 𝛾 𝑟𝑟 also enjoys an almost universally adopted approximation due to Pariser and Parr: