Potential profile in tunnel junction?

[chust]

The attached figure (and the following http link) is a schematic picture of my question.
http://image.baidu.com/detail/newindex?col=&tag=&pn=0&pid=32496347084&aid=411828503&user_id=113241369&setid=-1&sort=0&newsPn=&star=&fr=&from=2

Short version:
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How to calculate the electrostatic potential profile in the metal-vacuum-insulator-metal junction with a bias potential?
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Longer version:
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Hello every one,
I want to figure the energy level alignment between the molecule and the metal leads in a STM with a finite bias voltage ~2volts.  The molecule is decoupled from the metal substrate by an insulating decoupling layer (several monolayer of NaCl)   Since the two leads are biased, there must be a potential drop across the junction.  I guess it's crucial to know the potential profile to determine the energy level alignment between the molecule and the metal lead. But how can I calculate this electrostatic potential profile when the two leads are biased?
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Thanks very much for your help ^_^

[Daniele Stradi]

Dear Chust,

please look at the section "Voltage Drop" (an alternative name for the electrostatic potential drop) in this tutorial:
http://docs.quantumwise.com/tutorials/atk_transport_calculations.html?highlight=potential%20drop