You would simulate oxide tunneling. At some voltage stress (around >5MV/cm) electrons will start to tunnel through the oxide. This charge injected into the oxide will accumulate damage which will eventually destroy the oxide. The amount of charge injected to cause the oxide to breakdown is called the Qbd. For a very good oxide this is normally around 10 Coulombs. For a poor oxide around 0.1C. Silvaco has no way of knowing what this is but the stress at which tunneling occurs can be modeled. So if you know the Qbd and the time the oxide is under stress you can predict the breakdown.