OC20 multi-ads dataset (#1626)

* multi_ads placeholder docs

* add oc20-mads download link

Author: mshuaibii

docs/_toc.yml

@@ -51,6 +51,7 @@ parts:
   - file: catalysts/datasets/summary
     sections:
       - file: catalysts/datasets/oc20
+      - file: catalysts/datasets/oc20mads
       - file: catalysts/datasets/oc22
       - file: catalysts/datasets/oc20dense
       - file: catalysts/datasets/oc20neb

docs/catalysts/datasets/oc20_mads.md

@@ -0,0 +1,21 @@
+
+# Open Catalyst 2020 Multi-Adsorbate (mAds) Dataset
+
+## Overview
+The OC20-mAds dataset is a training set expanding the original OC20 dataset to include multi-adsorbate and coverage effects on catalyst surfaces. Adsorbates are randomly sampled from the list of OC20 adsorbates, up to 5 maximum adsorbates. For a small fraction of the dataset, all adsorbates on the surface may be identical. OC20-mAds is introduced in the [UMA paper](https://arxiv.org/pdf/2506.23971).
+## File Contents and Download
+|Splits |Size | MD5 checksum (download link)   |
+|---     |---    |---     |
+|Train   |   21,804,758   | [6435960ba5ad1a7c949bd2f2b51825bc](https://dl.fbaipublicfiles.com/opencatalystproject/data/oc20mAds/oc20_multiads_train.tar.gz)   |
+
+The following metadata can be accessed in the respective `atoms.info` entry:
+
+- `bulk_id`: Bulk identifier
+- `millers`: 3-tuple of integers indicating the Miller indices of the surface.
+- `shift`: C-direction shift used to determine cutoff for the surface (c-direction is following the nomenclature from Pymatgen).
+- `top`: Boolean indicating whether the chosen surface was at the top or bottom of the originally enumerated surface.
+- `adsorbates`: List of adsorbates sampled and their respective placements.
+- `sid`: Unique system identifier.
+- `fid`: Frame index along the relaxation/AIMD trajectory.
+- `results_path`: Internal results location.
+- `fmax`: Max per-atom force.