diff --git a/src/common.hpp b/src/common.hpp
index 43ba315..4328c58 100644
--- a/src/common.hpp
+++ b/src/common.hpp
@@ -1,49 +1,53 @@
 #ifndef SPECMICP_COMMON_HPP
 #define SPECMICP_COMMON_HPP
 
 #include <Eigen/Core>
 #include "boost/range/irange.hpp"
 
 namespace specmicp {
 
 //! type of an index
 using index_t = std::ptrdiff_t;
 using uindex_t = std::size_t;
 //! type of a scalar
 using scalar_t = double;
 
 // linear algebra
 //! A vector
 using Vector = Eigen::Matrix<scalar_t, Eigen::Dynamic, 1>;
 //! A matrix
 using Matrix = Eigen::Matrix<scalar_t, Eigen::Dynamic, Eigen::Dynamic>;
 
 //! \brief Range
 //!
 //! used to access all species of a certain types
 using range_t = boost::iterator_range<boost::range_detail::integer_iterator<index_t>>;
 
 
 // constants
 
 //! Id of an equation that is not an equation
 const index_t no_equation = -1;
 //! Id of a non-existant species
 const index_t no_species = -1;
 
+//! Precision used to compute jacobian
+const scalar_t eps_jacobian = 1e-8;
+
 } // namespace specmicp
 
 
 // macros
 // ------
 
+
 #ifdef SPECMICP_NO_DEBUG
 #define specmicp_assert
 #else
 #define specmicp_assert(x) assert(x)
 #endif // SPECMICP_NO_DEBUG
 
 
 
 #endif // SPECMICP_COMMON_HPP