diff --git a/include/cantera/thermo/PengRobinsonMFTP.h b/include/cantera/thermo/PengRobinsonMFTP.h
index 92880f70744..2d476742649 100644
--- a/include/cantera/thermo/PengRobinsonMFTP.h
+++ b/include/cantera/thermo/PengRobinsonMFTP.h
@@ -340,7 +340,7 @@ class PengRobinsonMFTP : public MixtureFugacityTP
      * Returns the number of solutions found. If it only finds the liquid
      * branch solution, it will return a -1 or a -2 instead of 1 or 2.  If it
      * returns 0, then there is an error.
-     * The cubic equation is solved using Nickall's method (http://www.nickalls.org/dick/papers/maths/cubic1993.pdf)
+     * The cubic equation is solved using Nickall's method (Ref: The Mathematical Gazette(1993), 77(November), 354–359, https://www.jstor.org/stable/3619777)
      */
     int NicholsSolve(double TKelvin, double pres, double a, double b, double aAlpha,
                      double Vroot[3]) const;
@@ -366,6 +366,7 @@ class PengRobinsonMFTP : public MixtureFugacityTP
     double m_a_current;
     double m_aAlpha_current;
 
+    // Vectors required to store a_coeff, b_coeff, alpha, kappa and other values for every species. Length = m_kk
     vector_fp a_vec_Curr_;
     vector_fp b_vec_Curr_;
     vector_fp aAlpha_vec_Curr_;