Molecular Candidate Ranking

molecular-candidate-ranking

Summary

Rank reference database molecules by similarity to CNN-predicted molecular embeddings to identify the most likely candidate structures. This skill bridges model prediction and molecule identification by filtering and ordering matches using distance metrics.

When to use

After a trained CNN model has generated molecular embeddings for query spectra, and you need to retrieve the most likely candidate molecules from a reference database. Use this skill when you have both predicted embeddings (from MS/MS spectra) and pre-computed reference database embeddings, and similarity scores alone are insufficient—you need a ranked, filtered list ordered by match confidence.

When NOT to use

• Query embeddings have not been generated or validated—embedding quality directly determines ranking validity.
• Reference database embeddings are stale or incompatible with the query embedding space (e.g., from a different CNN architecture or training run).
• The number of candidates required is extremely large (>1000) or the database is too small (<10 reference compounds), making ranking less discriminative.

Inputs

• CNN-predicted molecular embeddings for query spectra (vector format)
• Reference database embeddings (pre-computed vector representations)
• Similarity threshold parameter (float, typically 0.5–0.95 depending on metric)
• Distance metric specification (e.g., 'cosine_similarity' or 'euclidean')

Outputs

• Ranked candidate list (molecules sorted by decreasing similarity score)
• Match scores for each candidate (numeric, 0–1 or distance units)
• Reference molecule identifiers (e.g., InChI, compound names, database IDs)
• Filtered candidates CSV or structured table with top-N results

How to apply

Compute pairwise similarity scores between query embeddings and all reference database embeddings using a distance metric such as cosine similarity or Euclidean distance. Rank reference molecules in descending order by similarity score for each query compound. Apply a similarity threshold to retain only high-confidence matches, filtering out weak matches below the threshold. Generate a ranked candidate list annotated with match scores and reference molecule identifiers. The ranking and filtering steps are interdependent: the distance metric choice (cosine vs. Euclidean) directly affects which molecules rank highest, and the threshold acts as a hard cutoff to control candidate set size and false-positive rate.

Related tools

• Convolutional Neural Network (CNN) (Generates molecular embeddings from MS/MS spectra that serve as query vectors for similarity computation) — https://github.com/massspecdl/ChemEmbed
• ChemEmbed (End-to-end framework implementing the full pipeline from spectra preprocessing through CNN prediction to candidate matching and ranking) — https://github.com/massspecdl/ChemEmbed

Examples

python main.py --config config.yaml
# where config.yaml specifies top_n_candidates: 5, distance metric: cosine_similarity, similarity_threshold: 0.7

Evaluation signals

• Ranked list is sorted in descending order by similarity score with no inversions or ties inconsistently ordered.
• All candidates in the output exceed the specified similarity threshold; no candidates below threshold are retained.
• Reference molecule identifiers in output match valid entries in the reference database pickle file (no orphaned or corrupted IDs).
• Match scores are within expected range (0–1 for cosine similarity, or non-negative for Euclidean distance) and are mathematically consistent with the distance metric used.
• Top-N candidates returned does not exceed the configured top_n_candidates parameter; list is truncated or padded consistently if fewer high-scoring matches exist.

Limitations

• Ranking quality depends critically on the quality and representativeness of reference database embeddings; sparse or biased reference databases will produce poor candidate sets even with high similarity scores.
• The choice of distance metric (cosine vs. Euclidean) significantly impacts ranking order; no universal metric works optimally across all embedding spaces or chemical domains.
• Threshold selection is data- and model-dependent; a fixed threshold may be too stringent on small databases or too permissive on large ones, and the README does not provide principled guidance for threshold tuning.
• The framework matches embeddings without considering chemical feasibility, synthetic accessibility, or domain-specific priors; a highly ranked candidate may be chemically implausible or unavailable.

Evidence

• [other] Compute pairwise similarity scores between query embeddings and all reference database embeddings using a distance metric (e.g., cosine similarity or Euclidean distance).: "Compute pairwise similarity scores between query embeddings and all reference database embeddings using a distance metric (e.g., cosine similarity or Euclidean distance)."
• [other] Rank reference molecules by similarity score in descending order for each query.: "Rank reference molecules by similarity score in descending order for each query."
• [other] Filter candidate matches by applying a similarity threshold to retain only high-confidence matches.: "Filter candidate matches by applying a similarity threshold to retain only high-confidence matches."
• [readme] Matches predicted embeddings with a reference database to find top candidate molecules based on cosine similarity.: "Matches predicted embeddings with a reference database to find top candidate molecules based on cosine similarity."
• [readme] top_n_candidates: Number of top candidate molecules to retrieve from the reference database. (default: 5): "top_n_candidates: Number of top candidate molecules to retrieve from the reference database. (default: 5)"