ADDED   mttroot/mtt/lib/cr/r/cp.cr
Index: mttroot/mtt/lib/cr/r/cp.cr
==================================================================
--- mttroot/mtt/lib/cr/r/cp.cr
+++ mttroot/mtt/lib/cr/r/cp.cr
@@ -0,0 +1,27 @@
+%SUMMARY cp    Chemical potential constitutive relationship
+
+% This formula gives the Gibb's free energy for a reaction component
+% with given concentration.
+% k_e is the corresponding equilibrium constant.
+
+% C version integral causality
+FOR ALL  concentration, k_e, RT
+LET cp(C,k_e,RT, effort, 1, 
+        concentration, state, 1)
+         = RT*log(k_e*concentration);
+
+% C version derivative causality
+FOR ALL  potential, k_e, RT
+LET cp(C,k_e,RT, state, 1, 
+        potential, effort, 1)
+         = exp(potential/RT)/k_e;
+
+%AE version
+FOR ALL  concentration, RT
+LET cp(AE,RT,effort,2,concentration,effort,1) 
+        = RT*log(concentration);
+
+%AE version with k_e
+FOR ALL  concentration, k_e, RT
+LET cp(AE,k_e,RT,effort,2,concentration,effort,1) 
+        = RT*log(k_e*concentration);