Smooth shading/ smooth triangulation

I want to render scene with smooth triangles, not flat triangles.
So i am providing the geometry with primitive triangles with 3 vertices and 3 vertices normals.

But no matter what I am doing, I will always get flat triangles.
I am using the intersection and bounding box program provided by triangle_mesh.cu, basically everything provided by the sample6. The vertices and normal buffers + vertices/normal indices buffers are set correctly and the phong shade is the default shader in this program i guess.

So how can I get a smooth triangulation ? Do I have to change the intersection/ bounding box program?

triangle_mesh.cu supports smooth shading if the normal_buffer is filled and the indices are not negative.
Search for “normal_buffer” and you’ll find the code calculating the “shading_normal” attribute.

There was an issue with OBJ files and relative indices in the past which has been fixed in OptiX 3.8.0.
https://devtalk.nvidia.com/default/topic/817547/optix/rendering-with-smooth-normals/