NPT MD with Z-axis relaxation only (MD constraints)

[beomjin]

Hi all,

I’m building an interface between two materials in two steps:
(1) Use the interface builder and relax the interface structure.
(2) Perform 1200K 10ps MD annealing to strengthen interfacial bonding (interface stabilization).

Question: In step (2), I want to run NPT MD with the XY lattice fixed while atoms and the Z axis can relax (method: Martyna–Tobias–Klein). As an alternative, I am currently running NVT with Z-axis relaxation.

I only want the Z lattice to relax, so I tried enabling zz pressure coupling only and disabling xx/yy in the barostat with 0 bar and Isotropic pressure. However, during MD the XYZ lattice all relax.

Is there any way to relax only Z in MD, or is this not supported in Q-ATK? (VASP supports this by ICONST.)

Thanks

[AsifShah]

hi
Check under constraints in MD and optimization block and use tags for atoms you want to constraint.

[beomjin]

Hi,

In the OptimizeGeometry block, I can constrain the cell along the X, Y, and Z directions to fix each lattice constant during relaxation. However, in the MD block, there's no such an option.

Just to clarify, my question is about constraining the lattice constants, not about fixing individual atoms by tagging.

[AsifShah]

Hi,
In MD block, you can then use NVT ensemble if you want to fix cell vectors. If you use NPT it will allow volume to change during MD.

[beomjin]

I am currently using NVT Nose–Hoover for MD annealing, followed by atom and Z-axis relaxation with X and Y fixed.

With this approach, I observe about a 3% reduction in the Z lattice constant after relaxation, likely due to the relatively loose bonding in the initially relaxed interface structure. This means the cell experiences ~3% compressive strain along the Z direction during NVT MD, which may act as an artifact caused by the NVT XYZ constraint.

Since this strain likely arises from the use of NVT, NPT with Z-axis relaxation would provide a more realistic description of interface annealing, but this option is not supported.

[AsifShah]

What I understand is that you want to do NPT while fixing X and Y directions.
I think this is not possible.
Instead, what you can do is to use a NVT ensemble with vacuum in z direction. This will allow your system to relax in z direction but again this has some artifacts. One is that there will be surface effects at the termination in z direction. And one way to avoid that is using rigid atom constraint there.