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Fix numerical roundoff errors in the analytic solution of association site fractions for asymmetric association schemes #393

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Merged
prehner merged 1 commit into from
Oct 13, 2026
Merged

Fix numerical roundoff errors in the analytic solution of association site fractions for asymmetric association schemes #393

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3 changes: 3 additions & 0 deletions CHANGELOG.md
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Expand Up @@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0

## [Unreleased]

### Fixed
- Fixed numerical roundoff errors in the analytic solution of association site fractions for asymmetric association schemes. [#393](https://github.com/feos-org/feos/pull/393)

## [0.10.2] - 2026-10-12
### Fixed
- Fixed off-by-one error for the convergence-check in `_density_iteration`. [#386](https://github.com/feos-org/feos/pull/386)
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17 changes: 12 additions & 5 deletions crates/feos/src/association/dft.rs
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@@ -1,6 +1,6 @@
use super::*;
use feos_core::parameter::GenericParameters;
use feos_core::FeosResult;
use feos_core::parameter::GenericParameters;
use feos_dft::{FunctionalContribution, WeightFunction, WeightFunctionInfo, WeightFunctionShape};
use ndarray::{Array1, Array2, ArrayBase, ArrayView2, Axis, Data, Ix1, Slice};
use num_dual::DualNum;
Expand Down Expand Up @@ -297,10 +297,17 @@ impl<'a, A: AssociationStrength> YuWuAssociationFunctional<'a, A> {
* self.model.association_strength_ij(temperature, i, j, par);

// no cross association, two association sites
let aux = &delta * (&rhob - &rhoa) + 1.0;
let xa = ((&aux * &aux + &delta * &rhoa * 4.0).map(N::sqrt) + &aux).map(N::recip) * 2.0;
let aux = -aux + 2.0;
let xb = ((&aux * &aux + delta * &rhob * 4.0).map(N::sqrt) + aux).map(N::recip) * 2.0;
let (xa, xb) = if rhoa.sum().re() > rhob.sum().re() {
let aux = &delta * (&rhoa - &rhob) + 1.0;
let xb = ((&aux * &aux + &delta * &rhob * 4.0).map(N::sqrt) + aux).map(N::recip) * 2.0;
let xa = (&delta * &rhob * &xb + 1.0).map(N::recip);
(xa, xb)
} else {
let aux = &delta * (&rhob - &rhoa) + 1.0;
let xa = ((&aux * &aux + &delta * &rhoa * 4.0).map(N::sqrt) + aux).map(N::recip) * 2.0;
let xb = (&delta * &rhoa * &xa + 1.0).map(N::recip);
(xa, xb)
};

let f = |x: N| x.ln() - x * 0.5 + 0.5;
rhoa * xa.mapv(f) + rhob * xb.mapv(f)
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11 changes: 9 additions & 2 deletions crates/feos/src/association/mod.rs
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Expand Up @@ -152,8 +152,15 @@ impl Association {

// fraction of non-bonded association sites
let sqrt = ((delta * (rhoa - rhob) + 1.0).powi(2) + delta * rhob * 4.0).sqrt();
let xa = (sqrt + (delta * (rhob - rhoa) + 1.0)).recip() * 2.0;
let xb = (sqrt + (delta * (rhoa - rhob) + 1.0)).recip() * 2.0;
let (xa, xb) = if rhoa > rhob {
let xb = (sqrt + delta * (rhoa - rhob) + 1.0).recip() * 2.0;
let xa = (delta * rhob * xb + 1.0).recip();
(xa, xb)
} else {
let xa = (sqrt + delta * (rhob - rhoa) + 1.0).recip() * 2.0;
let xb = (delta * rhoa * xa + 1.0).recip();
(xa, xb)
};

rhoa * (xa.ln() - xa * 0.5 + 0.5) + rhob * (xb.ln() - xb * 0.5 + 0.5)
}
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