$piezcharge

piezcharge defines the polarization charge density difference between each layer. If there are 10 layers then we have 9 interfaces. So, we only need to set 9 regions.

Example

$piezcharge 
 0.00000e20  This means the polarization difference between layer 1 and 2 is 0.000. 
 1.00000e13  This means the polarization difference between layer 2 and 3 is 1.00000e13. It looks like positive interface charges. 
-1.00000e13  This means the polarization difference between layer 3 and 4 is -1.00000e13. It looks like negative interface charges.
 1.00000e13  This means the polarization difference between layer 4 and 5 is 1.00000e13. It looks like positive interface charges.
-1.00000e13  This means the polarization difference between layer 6 and 7 is 1.00000e13. It looks like positive interface charges.
 1.00000e13  This means the polarization difference between layer 7 and 8 is -1.00000e13. It looks like negative interface charges.
-1.00000e13  This means the polarization difference between layer 8 and 9 is 1.00000e13. it looks like positive interface charges.
 1.00000e13  This means the polarization difference between layer 9 and 10 is -1.00000e13. it looks like negative interface charges .
-1.00000e13