diff --git a/include/teqp/models/CPA.hpp b/include/teqp/models/CPA.hpp
index 6024546a633bb2ee5781d847f06153b5609d4819..4cfbd103572b78435542d1773e4dbf4ddada83f3 100644
--- a/include/teqp/models/CPA.hpp
+++ b/include/teqp/models/CPA.hpp
@@ -68,6 +68,8 @@ inline auto get_DeltaAB_pure(radial_dist dist, double epsABi, double betaABi, BT
 
 template<typename BType, typename TType, typename RhoType, typename VecType>
 inline auto XA_calc_pure(int N_sites, association_classes scheme, double epsABi, double betaABi, const BType b_cubic, const TType RT, const RhoType rhomolar, const VecType& molefrac) {
+    
+    using std::sqrt;
 
     // Matrix XA(A, j) that contains all of the fractions of sites A not bonded to other active sites for each molecule i
     // Start values for the iteration(set all sites to non - bonded, = 1)