Add vacuum in a device structure

[HIMANI]

Sir,
I have created a interface device and needed to optimise it. I want to give vacuum on both the sides of the device but not able to do so because the stretch cell option is not functional in case of device structures. I am using ATK 2014.1. Kindly suggest me a way to add vacuum.

[Jess Wellendorff]

Optimizing a device configuration implicitly means optimizing the central region. The electrodes are semi-infinite, and are therefore not supposed to change (they are supposed to be optimized before the device is created). Therefore, it makes no sense to add vacuum around the device. However, what you can do is to optimize the central region: Simply use a GeometryOptimization block. It will relax the central region only, while keeping the electrodes fixed as needed for NEGF calculations.

[Umberto Martinez]

I am using ATK 2014.1. Kindly suggest me a way to add vacuum.

Let me just add a note about updating to the newest ATL/VNL 2014 release which is the 2014.3 one.
download: http://quantumwise.com/products/download
list of updates with respect to previous ones: http://quantumwise.com/publications/quantumwise-news/item/894

[HIMANI]

I have optimized the central region i.e. the bulk structure before creating the device. Is it required to again optimize it?
Because I have tried optimising the whole device and it is already 15 days and it is still running.
What can I do in this context?

[HIMANI]

I want to know that if I do not optimize the device and perform Device density of states calculation over it. How it will effect the accuracy of the results?
The Bulk structure has been optimized before creating the device.

[Jess Wellendorff]

It is impossible to say how a Device DOS will be affected by some unknown measure of missing geometry optimization. Will be highly system dependent. Only way to determine this is by trying it out: DDOS for a relaxed device vs. DOS for some unrelaxed device.

Device geometry optimization can be rather painful. You say you have created an interface. Before creating the device, did you only optimize the individual bulks, or have you also separately optimized the central region of the interface device?

[HIMANI]

I have optimized the individual bulks  and then separately optimized the central region after interface formation also.

[Jess Wellendorff]

It sounds like you are pretty much ready for the DDOS calculation with no further optimization.

[HIMANI]

Thank you Sir!!