I made a cube of 1*1*1micomerter (ZnO)and apply 1MP pressure on top and bottom is fix  , 210 MP Young's modulus ,10.2 relative permittivity ,piezo electric stress 1.16*10^(-5),and after simulation I got 1.30 volt output in open circuit and 8.57*10^(-10)pC charge ?which equations are used by software to calaulate charge and voltage