Is AdsorbateSiteFinder dependent on the chosen unit cell


I am plan to use AdsorbateSiteFinder on two matched surface slabs to construct the potential energy surface (PES) of the interface.

I would like to first find only the unique sites of the two slabs, and actually compute the energy at all combinations of these points (e.g. on-top_1 / hollow_1, on-top_2 / hollow_1, …)

I already figured out a way to then match the unique points with all the points (returned when symm_reduce = 0). I then have the full unit cell filled with data and can interpolate to get a smooth PES.

In testing this I played around with graphene on Ni111 because of the good match and found some unexpected things:

  1. I find too many unique points for graphene (mp-1040425), while I get the correct amount for graphite (mp-48). Could this be due to there being only one layer in graphene? I see that the unit cells are different, and in this post @Joseph_Montoya explains that sometimes too many sites are listed, but I am a little confused nevertheless why the results are different for the two cells. Here are some pictures to illustrate with graphene on the top (note that bridge_5 is apparently obscured behind another bridge site) and graphite on the bottom:

Gr_unique Graphite_unique

  1. I am also surprised that the number of unique points changes based on the unit cell of Ni111. If I construct a slab out of the equilibrium fcc structure, i correctly find 1 on-top, 2 hollows, and 1 bridge site. However, if I match the cells and get a new rectangular cell, I get an additional bridge site which seems identical. Could this be due to the small strain that is imposed on the cell during the matching with the graphene cell?:

Simple_Ni111_HS Ni_matched_hsp

  1. If indeed the two bridge sites found in the matched cell are not equivalent, is there an option of telling AdsorbateSiteFinder to consider them equivalent? I tested slowly varying both symm_reduce from 0 to 1 and near_reduce from 0 to 1 while keeping the other value at the default of 0.01. I found that for both parameters, a value between 0.26 and 0.33 results in the correct number of sites for the unique points, but when I try to look at all points (setting symm_reduce=0 and near_reduce=0.30; right picture below) I am missing 4 of the bridge-sites in the middle of the cell (compared to setting symm_reduce=0 and near_reduce=0.01; left picture below).

Ni_0.01_all Ni_0.33_all

  1. My main questions are probably:
    • What do symm_reduce and near_reduce actually do? Why are the results the same if I change the two parameters independently? The documentation is unfortunately not very helpful there…
    • Is there a dependence on the chosen unit cell or am I experiencing some effect of small strains that are recognized by the algorithm?

Thanks for any help, if needed I can also provide some more context or code that I am using!

Cheers, Michael