Blender 2.79 | Cycles Renderer
The most realistic way to create real world metal shaders is to use Complex Fresnel reflection.
Cycles has a general implementation of a Complex Fresnel reflection in its Principled shader (when Metallic is set to 1.0), but this implementation doesn’t allow using real world physical numeric Complex IOR values in order to accurately render physical metals.
You can use a Complex IOR OSL shader such as this one from Chaos Group,
But there are some limitations with it:
1) It isn’t supported in GPU rendering.
2) For some reason I don’t know I couldn’t get it to work with Cycles..
Seeing these limitations I decided to develop a Complex Fresnel/IOR texture for Cycles that will work on GPU, and your welcome to download it here on my studio’s website:
The blend file itself contains a text with some Complex IOR preset values for metals,
And you can get more physical IOR data from refractiveindex.info