The
root cause of the simulation issue is that the file name in IPexpress (which is also the name of the IP's top module)
is the same as the name of the simulation model of the DDR2 memory.
So with two modules having the same name, only the module compiled later is loaded into the simulation library. When the simulation tool cannot find the library for the module compiled earlier, the mismatch error occurs.
To avoid this, the file entered in IPexpress should be renamed. This also applies to other IPs such as DDR and DDR3, which have simulation models.