SPICE piezo actuator

For a reasonable aproximation measure the capacitance between the piezos terminals and use a capacitor of the same value. You might also want to add back to back zener diodes whose zener voltage equals the breakdown voltage of the piezo element. Otherwise you will need to measure the impedance curve of the specific device you are working with and model it as a parallel set of series resonators of various Qs that provide the same results. A very tedious job... (accurately modeling electro-mechanical devices can be really messy.)