No external components should be added to the clock. There is an internal 30pf cap at each xtalin/xtalout pin. In an AC analysis, this will give you an effective 15pf capacitive load. There will typically be 2-3pf of stray cap in the logic. Therefore, the total capacitive load that an oscillating element sees is about 18pf. The 18pf load cap is a common recommended load cap for many oscillators in the market. So, you should choose a crystal that requires an external 18pf load, but do not supply these caps because they are integrated on chip.