How to add ambient occlusion pass for further post-processing?
Quick Tip: Ambient Occlusion Pass
To add an ambient occlusion pass to your rendering, go to Render Setup > Render Elements and click the "Add..." button. Select "CTexmap" render element and click "OK". In Render Elements window, scroll down to Texmap Render Element settings and click on the "None" button. Find CoronaAO map and load it. Drag-and-drop the loaded CoronaAO map into an empty slot in Material Editor to tune ambient occlusion map's parameters.
Note: you can also render other maps using CTexmap render element. For general info on the CTexmap, see: What is CTexmap render element?