A further iteration, where I've added an interface that maintains a

current mole fraction vector.

Cantera/src/thermo/GibbsExcessVPSSTP.cpp

@@ -9,7 +9,7 @@
  * variable pressure standard state methods for calculating
  * thermodynamic properties that are further based upon expressions
  * for the excess gibbs free energy expressed as a function of
- * the mole fractions
+ * the mole fractions.
  */
 /*
  * Copywrite (2009) Sandia Corporation. Under the terms of 
@@ -59,6 +59,7 @@ namespace Cantera {
     if (&b != this) {
       VPStandardStateTP::operator=(b);
     }
+    moleFractions_       = b.moleFractions_;
     return *this;
   }
 
@@ -86,6 +87,45 @@ namespace Cantera {
    *  -------------- Utilities -------------------------------
    */
 
+  void GibbsExcessVPSSTP::setMassFractions(const doublereal* const y) {
+#if DEBUG_MODE
+    checkMFSum(y);
+#endif
+    State::setMassFractions(y);
+    getMoleFractions(DATA_PTR(moleFractions_));
+  }
+
+  void GibbsExcessVPSSTP::setMassFractions_NoNorm(const doublereal* const y) {
+#if DEBUG_MODE
+    checkMFSum(y);
+#endif
+    State::setMassFractions_NoNorm(y);
+    getMoleFractions(DATA_PTR(moleFractions_));
+  }
+
+ void GibbsExcessVPSSTP::setMoleFractions(const doublereal* const x) {
+#if DEBUG_MODE
+    checkMFSum(x);
+#endif
+    State::setMoleFractions(x);
+    getMoleFractions(DATA_PTR(moleFractions_));
+  }
+
+  void GibbsExcessVPSSTP::setMoleFractions_NoNorm(const doublereal* const x) {
+#if DEBUG_MODE
+    checkMFSum(x);
+#endif
+    State::setMoleFractions_NoNorm(x);
+    getMoleFractions(DATA_PTR(moleFractions_));
+  }
+
+
+  void GibbsExcessVPSSTP::setConcentrations(const doublereal* const c) {
+    State::setConcentrations(c);
+    getMoleFractions(DATA_PTR(moleFractions_));
+  }
+
+
   // Equation of state type flag.
   /*
    * The ThermoPhase base class returns
@@ -148,6 +188,15 @@ namespace Cantera {
     return 0;
   }
 
+  double GibbsExcessVPSSTP::checkMFSum(const doublereal * const x) const {
+    doublereal norm = accumulate(x, x + m_kk, 0.0);
+    if (fabs(norm - 1.0) > 1.0E-9) {
+      throw CanteraError("GibbsExcessVPSSTP::checkMFSun",
+			 "MF sum exceeded tolerance of 1.0E-9:" + fp2str(norm));
+    }
+    return norm;
+  }
+
   /*
    * Returns the units of the standard and general concentrations
    * Note they have the same units, as their divisor is 

Cantera/src/thermo/GibbsExcessVPSSTP.h

@@ -312,6 +312,8 @@ namespace Cantera {
      * @{
      */
 
+
+
     //@}
 
     /**
@@ -321,6 +323,66 @@ namespace Cantera {
      * @{
      */
 
+    /**
+     * Set the mass fractions to the specified values, and then
+     * normalize them so that they sum to 1.0.
+     * @param y Array of unnormalized mass fraction values (input).
+     * Must have a length greater than or equal to the number of
+     * species.
+     *
+     * @param y  Input vector of mass fractions.
+     *           Length is m_kk.
+     */
+    virtual void setMassFractions(const doublereal* const y);
+
+    /**
+     * Set the mass fractions to the specified values without
+     * normalizing. This is useful when the normalization
+     * condition is being handled by some other means, for example
+     * by a constraint equation as part of a larger set of
+     * equations.
+     *
+     * @param y  Input vector of mass fractions.
+     *           Length is m_kk.
+     */
+    virtual void setMassFractions_NoNorm(const doublereal* const y);
+
+
+    /**
+     * Set the mole fractions to the specified values, and then
+     * normalize them so that they sum to 1.0.
+     * @param x Array of unnormalized mole fraction values (input).
+     * Must have a length greater than or equal to the number of
+     * species.
+     *
+     * @param x  Input vector of mole fractions.
+     *           Length is m_kk.
+     */
+    virtual void setMoleFractions(const doublereal* const x);
+    /**
+     * Set the mole fractions to the specified values without
+     * normalizing. This is useful when the normalization
+     * condition is being handled by some other means, for example
+     * by a constraint equation as part of a larger set of
+     * equations.
+     *
+     * @param x  Input vector of mole fractions.
+     *           Length is m_kk.
+     */
+    virtual void setMoleFractions_NoNorm(const doublereal* const x);
+
+    /**
+     * Set the concentrations to the specified values within the
+     * phase.
+     *
+     * @param c The input vector to this routine is in dimensional
+     *        units. For volumetric phases c[k] is the
+     *        concentration of the kth species in kmol/m3.
+     *        For surface phases, c[k] is the concentration
+     *        in kmol/m2. The length of the vector is the number
+     *        of species in the phase.
+     */
+    virtual void setConcentrations(const doublereal* const c);
 
     //@}
 
@@ -392,6 +454,10 @@ namespace Cantera {
      */
     doublereal err(std::string msg) const;
 
+  protected:
+
+    double checkMFSum(const doublereal * const x) const;
+
   private:
 
     //! Storage for the current values of the mole fractions of the species