Hi Ronghui,
I looked at this briefly. I agree that the initial density operator does not look right. If spins 1 and 2 come from parahydrogen the initial density operator should simply be opI[1].opI[2].
In general, when asking for advice, it is not constructive to say "something is wrong". Please be more helpful and specific, and provide a highlighted comment in the .nb file.
good luck
malcolm