Is the negative current meaningful?

[Nordland]

It is strictly not needed, but it is worth checking if it has an influence on the calculate current.
I would personally do it, but again it is in many case not needed.

[frsy]

Dear Nordlan, thank you!
I will do the check! 
BTW: Is the calculated current is related to the two-probe charge "q"? I meant if "q" is not changed when I tuned the parameter will the current be not changed also?

[Nordland]

The current and q is unrelated - however in these sense that if q changes it is very likely the that the current changes. However in many cases q does not change, but the current do...

[frsy]

It's clear. Thank you, Nordland!