overlap integral, \(S_{\text{rs}}\)

https://doi.org/10.1351/goldbook.O04357
Integral over space of the type ∫ψr*.ψs.dτ where ψr and ψs are different wavefunctions.
Source:
Green Book, 2nd ed., p. 17 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)