the transimission spectrum in a two probe system

[Nordland]

Agreed that something is fishy - it should be the most easy to converge it under 0 V bias. I have never seen this before, where a system could converge under finite bias, but not under zero bias.

[yangzw1985]

But the question is just like that!!
I also can not find out the reason. All of my two probe model was made by the same methods.
Now, there is something strange occured.
The zno nanostructure with 72 and 144 atoms convergenced normally under applied voltages of 0 V. While the zno nanostructure with 48, 96 and 120 atoms can not convergence under 0V.
I am really confusing!!
what should I do to solve the problem?