Analysing Tank

Hi,

I'm trying to analyse a SS316L cylindrical tank, using Ansys software for optimising its thickness. The tank would be subjected to 600mm Hg of vacuum. Considering the worst condition to be as complete vacuum, i need to apply pressure on the tank in the software. I'm not sure as to where should i apply the pressure on the tank, would that be minus atmospheric pressure inside alone, or atmospheric pressure on the outer surface of the tank, or both of them.

In normal pressure tank, say filled completely with water with certain pressure, i suppose we apply only inside pressure of the water and atmospheric pressure is not applied on the outside surface while analysing it in the software.

I hope people must have analysed vacuum tank, can someone let me know the correct procedure.

Thanks !