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Thermostat issues when simulating surface bombardment
Date: 2021/11/04 23:26
Name: Pavel Ondra&#269;ka   <pavel.ondracka@email.cz>

Dear OpenMX users and developers,

I want to simulate a bombardment of a surface slab with single atoms. The general idea seems to work fine, however I have some problems with the temperature control via thermostat. I want to run at ~300K, so I take the slab, turn on Nos&#233;-Hoover thermostat and let it thermalize. Next step I take the final dat# file, I introduce Ti or N atom few &#197; from the surface with some defined initial velocity (energy). Now the problem is, before it hits the surface, it is already affected by the thermostat so its velocity and direction at the time it collides is not well defined.

I'm not sure how to proceed now, I was reading the UG but it seems it is not possible to remove some atom from the temperature control. The simple solution might be to switch to NVE after thermalization, however I want to actually repeat the process multiple times, so the surface would be heating up without the temperature control. I thought about switching back and forth to NVE before the atom hits the surface and than back to NVT_NH to thermalize again, but that will probably prolong the simulation unnecessarily (as I will have to wait for the stabilization again, and I'm not sure what happens to the thermostat internal state)? Other option would be to try to introduce the atom as close to the surface as possible to reduce the thermostats influence as much as possible before it collides (and/or try to increase the heat bath mass, to not have so tight temperature control). But that has its own problems as my surface is not very smooth.

Is there some better way I have not thought of? I'd be grateful for any ideas.

Best regards
Pavel
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