difference between optimization using device LCAO and LCAO for bulk

[pshinyeong]

Hello,

I have a question about the best approach for relaxing a device/bulk configuration. Specifically, my system is a vacuum/metal (6 layers)/barrier (6 layers)/metal (6 layers)/vacuum structure. I want to optimize only two layers on each side of the interfaces (i.e., four layers total at each interface) while keeping the other layers fixed.

My ultimate goal is to calculate transmission, so eventually I need to convert this bulk configuration into a device configuration. However, I noticed that:

Before optimization, I can convert the bulk configuration into a device configuration without any issues.
After optimization, the optimized configuration cannot be converted into a device configuration.
Given this, I have two questions:

Is it more reasonable to create the device configuration first and then optimize it (using a Device LCAO Calculator), rather than optimizing the bulk configuration and then converting it into a device configuration?

Is there any difference between these two methods in terms of results or best practices?

Any guidance would be greatly appreciated. Thank you!

[Anders Blom]

In terms of results both methods should be similar, but using bulk relaxation before you convert to device is faster. What you need to do, in order to still  be able to convert the structure to a device, is to keep the atoms that will not relax fully fixed. This needs to be enough layers that the periodicity of the structure is still maintained. So, if there are 3 layers in the electrode (before relaxation) you need to keep 4 layers fixed, so that the algorithms notices that layers 1 and 4 are identical.