There are a couple of different ways.
Simplest is perhaps to load the system into the Atomic Manipulator in VNL. Then, you click the button "Save Equivalent Bulk...".
There is also a NanoLanguage command; once you have created a two-probe system, the returned object has a method called
equivalentBulkSystem(). Here's a very sketchy example of how to use it:
from ATK.TwoProbe import *
...
twoprobe = TwoProbeConfiguration (...)
equiv_bulk = twoprobe.equivalentBulkSystem()
For reference: The equivalent bulk system contains the electrodes plus the central region, represented as a
PeriodicAtomConfiguration. This is the system calculated by ATK in the "equivalent bulk run" that precedes the two-probe calculation when using the (default) keyword
InitialDensity.EquivalentBulk in
twoProbeAlgorithmParameters().
Hope that helps!