Chris Quan
Ansys Employee

The EOS (Equation Of State) in Autodyn defines the relation between the hydrostatic pressure, the local density, and local specific energy.

Thus, every EOS, including user-defined EOS, should calculate the current pressure from the current internal energy, update the internal energy for pressure calculation in the next time step, and finally update the material sound speed based upon the current condition (pressure, density, energy, etc).