Fix isothermal compressibility calculation (#342)

RolfStierle · web-flow · commit 89f81f77af55 · 2026-03-02T14:44:38.000+01:00
* Fix isothermal compressibility calculation

Comment includes negative sign, calculation doesn't. Results in negative values for the calculation of isothermal_compressibility().

* CHANGELOG updated
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Changed
 - Automatically detect VLLEs in `BinaryPhaseDiagram`. [#341](https://github.com/feos-org/feos/pull/341)
+- Correct missing minus sign for `isothermal_compressibility`. [#342](https://github.com/feos-org/feos/pull/342)
 
 ## [0.9.3] - 2026-01-26
 ### Added
diff --git a/crates/feos-core/src/state/residual_properties.rs b/crates/feos-core/src/state/residual_properties.rs
@@ -272,7 +272,7 @@ where
 
     /// Isothermal compressibility: $\kappa_T=-\frac{1}{V}\left(\frac{\partial V}{\partial p}\right)_{T,N_i}$
     pub fn isothermal_compressibility(&self) -> InvP<D> {
-        (self.dp_dv(Contributions::Total) * self.volume).inv()
+        -(self.dp_dv(Contributions::Total) * self.volume).inv()
     }
 
     // entropy derivatives

Original file line number	Diff line number	Diff line change
`@@ -272,7 +272,7 @@ where`
`272`	`272`
`273`	`273`	`/// Isothermal compressibility: $\kappa_T=-\frac{1}{V}\left(\frac{\partial V}{\partial p}\right)_{T,N_i}$`
`274`	`274`	`pub fn isothermal_compressibility(&self) -> InvP<D> {`
`275`		`- (self.dp_dv(Contributions::Total) * self.volume).inv()`
	`275`	`+ -(self.dp_dv(Contributions::Total) * self.volume).inv()`
`276`	`276`	`}`
`277`	`277`
`278`	`278`	`// entropy derivatives`