https://doi.org/10.1351/goldbook.S06174
Defined by: \[H^{\sigma }=U^{\sigma }- \gamma \ A_{\text{s}}\] where \(U^{\sigma }\) is the @[email protected], \(\gamma \) is the @[email protected], and \(A_{\text{s}}\) is the area relative to a @[email protected]