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Updates to make HP queries return values instead of failing for higher pressures #2087

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e462467
updates to make HP queries return values instead of failing for highe…
wandadars Feb 13, 2026
2901fd9
address degenerate case of a=0 and b=0 for cubic EoS and added ideal …
wandadars Feb 23, 2026
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36 changes: 36 additions & 0 deletions include/cantera/thermo/MixtureFugacityTP.h
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Original file line number Diff line number Diff line change
Expand Up @@ -298,6 +298,42 @@ class MixtureFugacityTP : public ThermoPhase
*/
double z() const;

//! @name Ideal-Limit Utilities for Cubic EoS
//!
//! These helpers provide shared ideal-gas-limit expressions used by
//! cubic-EoS implementations when mixture non-ideal terms vanish.
//! @{

//! Molar heat capacity at constant pressure in the ideal-gas limit.
//! Units: J/kmol/K.
double cp_mole_ideal() const;

//! Molar heat capacity at constant volume in the ideal-gas limit.
//! Units: J/kmol/K.
double cv_mole_ideal() const;

//! Standard concentration in the ideal-gas limit for species @p k.
//! Units: kmol/m^3.
double standardConcentration_ideal(size_t k) const;

//! Ideal-gas-limit activity coefficients for all species.
//! Sets all entries of @p ac to 1.
void getActivityCoefficients_ideal(span<double> ac) const;

//! Ideal-gas-limit chemical potentials for all species.
//! @param mu Output vector of chemical potentials [J/kmol].
void getChemPotentials_ideal(span<double> mu) const;

//! Ideal-gas-limit partial molar enthalpies for all species.
//! @param hbar Output vector of partial molar enthalpies [J/kmol].
void getPartialMolarEnthalpies_ideal(span<double> hbar) const;

//! Ideal-gas-limit partial molar entropies for all species.
//! @param sbar Output vector of partial molar entropies [J/kmol/K].
void getPartialMolarEntropies_ideal(span<double> sbar) const;

//! @}

//! Calculate the deviation terms for the total entropy of the mixture from
//! the ideal gas mixture
/**
Expand Down
72 changes: 72 additions & 0 deletions src/thermo/MixtureFugacityTP.cpp
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Expand Up @@ -13,6 +13,8 @@
#include "cantera/base/utilities.h"
#include "cantera/base/global.h"

#include <algorithm>

using namespace std;

namespace Cantera
Expand Down Expand Up @@ -266,6 +268,60 @@ double MixtureFugacityTP::z() const
return pressure() * meanMolecularWeight() / (density() * RT());
}

double MixtureFugacityTP::cp_mole_ideal() const
{
return GasConstant * mean_X(m_cp0_R);
}

double MixtureFugacityTP::cv_mole_ideal() const
{
return GasConstant * (mean_X(m_cp0_R) - 1.0);
}

double MixtureFugacityTP::standardConcentration_ideal(size_t k) const
{
getStandardVolumes(m_workS);
return 1.0 / m_workS[k];
}

void MixtureFugacityTP::getActivityCoefficients_ideal(span<double> ac) const
{
checkArraySize("MixtureFugacityTP::getActivityCoefficients_ideal", ac.size(), m_kk);
for (size_t k = 0; k < m_kk; k++) {
ac[k] = 1.0;
}
}

void MixtureFugacityTP::getChemPotentials_ideal(span<double> mu) const
{
checkArraySize("MixtureFugacityTP::getChemPotentials_ideal", mu.size(), m_kk);
double RT_ = RT();
getStandardChemPotentials(mu);
for (size_t k = 0; k < m_kk; k++) {
double xx = std::max(SmallNumber, moleFraction(k));
mu[k] += RT_ * log(xx);
}
}

void MixtureFugacityTP::getPartialMolarEnthalpies_ideal(span<double> hbar) const
{
checkArraySize("MixtureFugacityTP::getPartialMolarEnthalpies_ideal", hbar.size(), m_kk);
getEnthalpy_RT_ref(hbar);
scale(hbar.begin(), hbar.end(), hbar.begin(), RT());
}

void MixtureFugacityTP::getPartialMolarEntropies_ideal(span<double> sbar) const
{
checkArraySize("MixtureFugacityTP::getPartialMolarEntropies_ideal", sbar.size(), m_kk);
getEntropy_R_ref(sbar);
scale(sbar.begin(), sbar.end(), sbar.begin(), GasConstant);
double logPres = log(pressure() / refPressure());
for (size_t k = 0; k < m_kk; k++) {
double xx = std::max(SmallNumber, moleFraction(k));
sbar[k] -= GasConstant * (log(xx) + logPres);
}
}

double MixtureFugacityTP::sresid() const
{
throw NotImplementedError("MixtureFugacityTP::sresid");
Expand Down Expand Up @@ -841,6 +897,12 @@ int MixtureFugacityTP::solveCubic(double T, double pres, double a, double b,
}

double tmp;
auto physicalRoot = [b](double v) {
// For cubic EoS, physically admissible roots satisfy V > b and V > 0.
double vmin = std::max(0.0, b * (1.0 + 1e-12));
return std::isfinite(v) && v > vmin;
};

// One real root -> have to determine whether gas or liquid is the root
if (disc > 0.0) {
double tmpD = sqrt(disc);
Expand Down Expand Up @@ -921,6 +983,11 @@ int MixtureFugacityTP::solveCubic(double T, double pres, double a, double b,
// Find an accurate root, since there might be a heavy amount of roundoff error due to bad conditioning in this solver.
double res, dresdV = 0.0;
for (int i = 0; i < nSolnValues; i++) {
if (!physicalRoot(Vroot[i])) {
// Non-physical roots (for example, negative molar volume) are not
// used for state selection and should not trigger convergence errors.
continue;
}
for (int n = 0; n < 20; n++) {
res = an * Vroot[i] * Vroot[i] * Vroot[i] + bn * Vroot[i] * Vroot[i] + cn * Vroot[i] + dn;
if (fabs(res) < 1.0E-14) { // accurate root is obtained
Expand All @@ -946,6 +1013,11 @@ int MixtureFugacityTP::solveCubic(double T, double pres, double a, double b,
"V = {}", T, pres, Vroot[i]);
}
}
if (nSolnValues == 1 && !physicalRoot(Vroot[0])) {
throw CanteraError("MixtureFugacityTP::solveCubic",
"single real root is non-physical for T = {}, p = {} "
"(V = {}, b = {})", T, pres, Vroot[0], b);
}

if (nSolnValues == 1) {
// Determine the phase of the single root.
Expand Down
77 changes: 74 additions & 3 deletions src/thermo/PengRobinson.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -10,10 +10,19 @@
#include "cantera/base/utilities.h"

#include <boost/algorithm/string.hpp>
#include <algorithm>

namespace Cantera
{

namespace
{
bool isIdealCubicLimit(double aMix, double bMix)
{
return aMix == 0.0 && bMix == 0.0;
}
}

const double PengRobinson::omega_a = 4.5723552892138218E-01;
const double PengRobinson::omega_b = 7.77960739038885E-02;
const double PengRobinson::omega_vc = 3.07401308698703833E-01;
Expand Down Expand Up @@ -83,6 +92,9 @@ void PengRobinson::setBinaryCoeffs(const string& species_i,
double PengRobinson::cp_mole() const
{
_updateReferenceStateThermo();
if (isIdealCubicLimit(m_aAlpha_mix, m_b)) {
return cp_mole_ideal();
}
double T = temperature();
double mv = molarVolume();
double vpb = mv + (1 + Sqrt2) * m_b;
Expand All @@ -97,6 +109,9 @@ double PengRobinson::cp_mole() const
double PengRobinson::cv_mole() const
{
_updateReferenceStateThermo();
if (isIdealCubicLimit(m_aAlpha_mix, m_b)) {
return cv_mole_ideal();
}
double T = temperature();
calculatePressureDerivatives();
return (cp_mole() + T * m_dpdT * m_dpdT / m_dpdV);
Expand All @@ -114,13 +129,16 @@ double PengRobinson::pressure() const

double PengRobinson::standardConcentration(size_t k) const
{
getStandardVolumes(m_workS);
return 1.0 / m_workS[k];
return standardConcentration_ideal(k);
}

void PengRobinson::getActivityCoefficients(span<double> ac) const
{
checkArraySize("PengRobinson::getActivityCoefficients", ac.size(), m_kk);
if (isIdealCubicLimit(m_aAlpha_mix, m_b)) {
getActivityCoefficients_ideal(ac);
return;
}
double mv = molarVolume();
double vpb2 = mv + (1 + Sqrt2) * m_b;
double vmb2 = mv + (1 - Sqrt2) * m_b;
Expand Down Expand Up @@ -154,8 +172,13 @@ void PengRobinson::getActivityCoefficients(span<double> ac) const

void PengRobinson::getChemPotentials(span<double> mu) const
{
getGibbs_ref(mu);
if (isIdealCubicLimit(m_aAlpha_mix, m_b)) {
getChemPotentials_ideal(mu);
return;
}
double RT_ = RT();

getGibbs_ref(mu);
for (size_t k = 0; k < m_kk; k++) {
double xx = std::max(SmallNumber, moleFraction(k));
mu[k] += RT_ * (log(xx));
Expand Down Expand Up @@ -189,6 +212,10 @@ void PengRobinson::getChemPotentials(span<double> mu) const

void PengRobinson::getPartialMolarEnthalpies(span<double> hbar) const
{
if (isIdealCubicLimit(m_aAlpha_mix, m_b)) {
getPartialMolarEnthalpies_ideal(hbar);
return;
}
// First we get the reference state contributions
getEnthalpy_RT_ref(hbar);
scale(hbar.begin(), hbar.end(), hbar.begin(), RT());
Expand Down Expand Up @@ -239,6 +266,11 @@ void PengRobinson::getPartialMolarEnthalpies(span<double> hbar) const

void PengRobinson::getPartialMolarEntropies(span<double> sbar) const
{
if (isIdealCubicLimit(m_aAlpha_mix, m_b)) {
getPartialMolarEntropies_ideal(sbar);
return;
}

// Using the identity : (hk - T*sk) = gk
double T = temperature();
getPartialMolarEnthalpies(sbar);
Expand Down Expand Up @@ -444,6 +476,9 @@ void PengRobinson::getSpeciesParameters(const string& name, AnyMap& speciesNode)

double PengRobinson::sresid() const
{
if (isIdealCubicLimit(m_aAlpha_mix, m_b)) {
return 0.0;
}
double molarV = molarVolume();
double hh = m_b / molarV;
double zz = z();
Expand All @@ -457,6 +492,9 @@ double PengRobinson::sresid() const

double PengRobinson::hresid() const
{
if (isIdealCubicLimit(m_aAlpha_mix, m_b)) {
return 0.0;
}
double molarV = molarVolume();
double zz = z();
double aAlpha_1 = daAlpha_dT();
Expand Down Expand Up @@ -519,6 +557,39 @@ double PengRobinson::densityCalc(double T, double presPa, int phaseRequested,
double volGuess = mmw / rhoGuess;
m_NSolns = solveCubic(T, presPa, m_a, m_b, m_aAlpha_mix, m_Vroot);

// Ensure root ordering used for branch selection only contains physical roots.
// This avoids selecting negative-volume roots in mixed-parameter mechanisms.
double vmin = std::max(0.0, m_b * (1.0 + 1e-12));
vector<double> physicalRoots;
for (double root : m_Vroot) {
if (std::isfinite(root) && root > vmin) {
physicalRoots.push_back(root);
}
}
std::sort(physicalRoots.begin(), physicalRoots.end());
if (physicalRoots.empty()) {
return -1.0;
} else if (physicalRoots.size() == 1) {
// Preserve branch-request semantics for single-root states:
// return -2 when the requested phase is inconsistent with the
// single branch identified by solveCubic() sign convention.
if ((phaseRequested == FLUID_GAS && m_NSolns < 0)
|| (phaseRequested >= FLUID_LIQUID_0 && m_NSolns > 0))
{
return -2.0;
}
// Otherwise, accept the physically admissible root directly.
return mmw / physicalRoots[0];
} else if (physicalRoots.size() == 2) {
m_Vroot[0] = physicalRoots[0];
m_Vroot[1] = 0.5 * (physicalRoots[0] + physicalRoots[1]);
m_Vroot[2] = physicalRoots[1];
} else {
m_Vroot[0] = physicalRoots[0];
m_Vroot[1] = physicalRoots[1];
m_Vroot[2] = physicalRoots[2];
}

double molarVolLast = m_Vroot[0];
if (m_NSolns >= 2) {
if (phaseRequested >= FLUID_LIQUID_0) {
Expand Down
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