how to set the length of center scattering region?

[fanjiaping]

    when i calculated the transmission of the device,there always are some mistakes through the process.For example,if there is much atom in the device,the calculation will stop. because the computer's momory can't afford. when I set less lays, the VNL shows me that the center scattering region is too short or too small.So
I am confused by this situation.could you give me some advices?
                                                   thank you!

[youngjfly]

add more surface atoms(same as electrode atoms)may help.

[zh]

Usually you should balance the accuracy and the required computational resource to choose the thickness of screening of surface layers. The surface layer  should be consistent with the periodicity  of the electrode according to the translational symmetry.