COSMO in 2D systems: boundary conditions definitions?

[cam]

Dear all,

I am trying to study the effect of functionalisation on monolayer MoS2 within a solvent (water). For that, I have added vacuum in the C direction to limit the interactions between layers. I would like to account for the screening effects of water through COSMO solvation model. However, I am struggling to find information about including this model for 2D systems. From my understanding:

1)  If I use 2D-FFT boundary conditions for A and B, and Dirichlet and Neumann for bottom and top, respectively, in the C direction (as suggested for bulk systems), my MoS2 slab will be modelled as the surface layer which continues into the bulk. Additionally, when I functionalise the material, my solvation surface is orientated in the wrong direction, regardless if I set Neumann at the top and Dirichlet at the bottom or vice versa, my solvation layer appears on the side of the slab where no functionalisation.

2) I have also tried to use Multipole boundary conditions with the conjugate gradient Poisson solver (suggested for charged molecules). However, I run into compiling errors.


Would you be able to advise on the correct boundary conditions to use in this case?
Thank you very much,

[BradWells]

Hi Cam,

COSMO is QATK is enabled for slab geometries. To set up a slab geometry it is assumed that the surface normal is aligned along the positive C direction. The COSMO surface is then placed on the surface that has the highest C value. Note that if you have a thin film like geometry, only one surface will be placed on the top side of the film. When setting up a slab this can be created using either a SurfaceConfiguration or a BulkConfiguration. In both cases Dirichlet/Neumann boundary conditions should be used along the C direction. Dirichlet can require that the potential is zero at the boundary, anchoring the to total potential to a specific value. Neumann can require that the potential gradient is zero at the boundary. These are both non-periodic boundary conditions and can be used with structures with an overall charge. The A and B directions can use periodic 2D-FFT boundary conditions. In the SurfaceConfiguration the slab is connected to the electrode, which models the extension of the material in the negative C direction. In a BulkConfiguration the bottom side of the surface needs to be terminated in a way so that the top solvated side is relatively unaffected by the termination. It is also recommended that enough vacuum be added on the top and bottom sides to allow the boundary conditions to be easily satisfied at the ends of the cell.

A thin film geometry with solvent on both sides of the surface can be represented as a fragment in a MoleculeConfiguration. Here the transverse directions of the film should be terminated so as not to significantly alter the electron density at the center of the fragment. In a MoleculeConfiguration the entire structure is surrounded by the COSMO surface, which in this case approximates solvent at both sides of the film. If the overall fragment (including any adsorbed molecules) is neutral, then the normal FFT boundary conditions can be used. If the overall fragment carries a charge, then Multipole boundary conditions should be used. The reason is that FFT boundary conditions assume a periodic potential, which is a good approximation for most neutral molecules. Charged molecules have additional long-range electrostatic interactions that are taken into account with the Multipole boundary condition.

I hope that helps you with your simulation.