Add missing function initialization

src/function.cpp

@@ -25,3 +25,9 @@ numerics::PolyWrapper numerics::functions::higher_parabola_1d =
     numerics::PolyWrapper({1., 0.0, -2.3, 0., -1.4});
 numerics::PolyWrapper numerics::functions::ref_polynomial_1d =
     numerics::PolyWrapper({1., -3., -1., 11., -17., 16., -4.});
+
+numerics::LennardJones numerics::functions::wide_LJ =
+    numerics::LennardJones(-4, 20);
+
+numerics::LennardJones numerics::functions::narrow_LJ =
+    numerics::LennardJones(-5, 3);