RuinaFunction: not differentiable for x < 1e-14

dune/tectonic/nicefunction.hh

@@ -55,7 +55,7 @@ class RuinaFunction : public NiceFunction {
 
   double virtual leftDifferential(double x) const {
     double const arg = eta_h * x;
-    if (arg == 0) // TODO: Make this controllable
+    if (arg < 1e-14) // TODO: Make this controllable
       return 0;
 
     double const ret = (a * std::log(arg) + c + K);
@@ -67,6 +67,9 @@ class RuinaFunction : public NiceFunction {
   }
 
   double virtual second_deriv(double s) const {
+    if (s < 1e-14) // TODO: Make this controllable
+      return 0;
+
     return coefficientProduct * a / s;
   }