resonance integral, H rs

In Hückel molecular orbital theory, the integral over space of the type $∫ ψ r * H ̂ ψ s ⁢ d τ$ where $H ̂$ is the hamiltonian operator and $ψ r$ and $ψ s$ are different wavefunctions.

Source:

Green Book, 2nd ed., p. 17