diff --git a/scripts/eval/classify_altloc_regions.py b/scripts/eval/classify_altloc_regions.py
index 0d1de1de..af29cda7 100644
--- a/scripts/eval/classify_altloc_regions.py
+++ b/scripts/eval/classify_altloc_regions.py
@@ -5,17 +5,18 @@
 
 1. ``side_chain_only`` : altloc atoms exist in the residue, but none of its
    backbone atoms have altlocs.
-2. ``small_loop`` : a contiguous backbone altloc span whose mean per-residue
-   backbone lDDT score (defined below) between altlocs is above
-   ``--loop-lddt-threshold`` (default 0.75).
-3. ``large_loop`` : a contiguous backbone-altloc span whose mean per-residue
-   backbone lDDT score between altlocs is below ``--loop-lddt-threshold``.
+2. ``small_loop`` : a contiguous backbone-altloc span whose loop score is above the threshold.
+3. ``large_loop`` : a contiguous backbone-altloc span whose loop score is below the threshold.
 4. ``domain_shift`` : a single contiguous backbone-altloc span longer than
-   ``--domain-shift-min-span`` residues (default 50). Classified before the
-   loop lDDT test.
+   ``--domain-shift-min-span`` residues (default 50). Classified before loop
+   scoring.
 
-Score definition (important, slightly different from canonical lDDT):
+Loop scores are calculated for every combination of altloc pairs when more than
+two altlocs are present. The selected scoring strategy defines how per-residue
+scores are reduced into a pair score, how pair scores are reduced into the final
+loop score, and how that loop score is thresholded.
 
+LDDT SCORER:
     For a given pair of altlocs, the score is the **equal-weighted arithmetic
     mean** of per residue backbone lDDT scores across the span:
 
@@ -32,19 +33,31 @@
     has the same neighbor count. The 0.75 default is calibrated for this specific
     calculation.
 
-Altloc pairing: when > 2 altlocs are present, the score above is
+RMSD SCORER:
+    For a given pair of altlocs, the score is the maximum of per residue
+    RMSD scores across the span:
+
+        score = max(score_k)
+
+    Each ``score_k`` is the standard all-atom RMSD from
+    :class:`sampleworks.metrics.rmsd.AllAtomRMSD`.
+
+`Altloc pairing`: when > 2 altlocs are present, the scores above are
 computed for every combination of altloc pairs and the span is
 classified by the *minimum* score over pair combinations.
 
-Use ``find_altloc_selections.py --min-span 1`` to ensure single-residue side only
+Use ``find_altloc_selections.py --min-span 1`` to ensure single-residue side chain only
 selections.
 """
 
 import argparse
 import json
+import operator
 import re
-from collections.abc import Mapping
+from collections.abc import Callable, Mapping
+from dataclasses import dataclass
 from pathlib import Path
+from typing import Any
 
 import numpy as np
 import pandas as pd
@@ -55,8 +68,10 @@
     ATOMWORKS_COMPARISON_OPS,
     get_mask_from_old_selection_string,
     parse_selection_string,
+    selection_to_residues,
 )
 from sampleworks.metrics.lddt import AllAtomLDDT
+from sampleworks.metrics.rmsd import AllAtomRMSD
 from sampleworks.utils.atom_array_utils import (
     BACKBONE_ATOM_TYPES,
     BLANK_ALTLOC_IDS,
@@ -77,10 +92,11 @@
     "end_res",
     "span_length",
     "classification",
-    "worst_pair_mean_backbone_lddt",
+    "score",
+    "score_metric",
     "n_backbone_altloc_residues",
     "n_altlocs",
-    "pair_lddts",
+    "pair_scores",
 ]
 
 
@@ -111,40 +127,85 @@ def _chain_from_selection(selection: str) -> str | None:
     return chain_id
 
 
-def _mean_residue_lddt_for_pair(
-    gt_array: AtomArray | AtomArrayStack | None,
-    pred_array: AtomArray | AtomArrayStack | None,
+@dataclass(frozen=True)
+class SpanScorer:
+    """Metric specific behavior for classification."""
+
+    metric_name: str
+    metric_compute: Callable[..., dict[str, Any]]
+    residue_scores_key: str
+    pair_score_reducer: Callable[[list[float]], float]
+    worse_pair_score: Callable[[list[float]], float]
+    is_small_loop_score: Callable[[float, float], bool]
+    default_threshold: float
+
+
+def _score_pair_with_scorer(
+    scorer: SpanScorer,
+    gt_array: AtomArrayStack | AtomArray | None,
+    pred_array: AtomArrayStack | AtomArray | None,
     chain: str,
     residues: list[int],
 ) -> float:
-    """Equal weighted arithmetic mean of per residue lDDT across the span."""
+    """Calculate a scorer's pair score over a residue span."""
     if gt_array is None or pred_array is None or not residues:
         return float("nan")
 
     res_clause = " or ".join(f"res_id == {r}" for r in residues)
     selection = f"chain_id == '{chain}' and ({res_clause}) and atom_name in ['C','CA','N','O']"
     try:
-        result = AllAtomLDDT().compute(
+        result = scorer.metric_compute(
             predicted_atom_array_stack=pred_array,
             ground_truth_atom_array_stack=gt_array,
             selection=selection,
         )
     except Exception as e:
-        logger.warning(f"lDDT compute failed for chain {chain} residues {residues}: {e}")
+        logger.warning(
+            f"{scorer.metric_name} computation failed for chain {chain} residues {residues}: {e}"
+        )
+        return float("nan")
+
+    residue_scores = result.get(scorer.residue_scores_key, {})
+    if not isinstance(residue_scores, dict):
+        logger.warning(
+            f"{scorer.metric_name} result did not contain a residue score dictionary "
+            f"under key '{scorer.residue_scores_key}'"
+        )
         return float("nan")
 
-    residue_scores = result.get("residue_lddt_scores", {})
     keys = [f"{chain}{r}" for r in residues]
     missing = [k for k in keys if k not in residue_scores]
     if missing:
         logger.warning(
-            f"lDDT result missing residues {missing} for chain {chain}. This means the result"
-            f"averaged only over the {len(keys) - len(missing)} residues it returned"
+            f"{scorer.metric_name} result missing residues {missing} for chain {chain}. "
+            f"This means the result was reduced only over the "
+            f"{len(keys) - len(missing)} residues it returned"
         )
-    flat = [residue_scores[k][0] for k in keys if k in residue_scores]
-    if not flat:
-        return float("nan")
-    return float(np.mean(flat))
+
+    flat = [float(residue_scores[k][0]) for k in keys if k in residue_scores and residue_scores[k]]
+    return float(scorer.pair_score_reducer(flat)) if flat else float("nan")
+
+
+LOOP_SCORERS = {
+    "lddt": SpanScorer(
+        metric_name="lddt",
+        metric_compute=AllAtomLDDT().compute,
+        residue_scores_key="residue_lddt_scores",
+        pair_score_reducer=np.mean,
+        worse_pair_score=min,
+        is_small_loop_score=operator.gt,
+        default_threshold=0.75,
+    ),
+    "rmsd": SpanScorer(
+        metric_name="rmsd",
+        metric_compute=AllAtomRMSD(superimpose=False).compute,
+        residue_scores_key="residue_rmsd_scores",
+        pair_score_reducer=max,
+        worse_pair_score=max,
+        is_small_loop_score=operator.lt,
+        default_threshold=1.0,
+    ),
+}
 
 
 def _classify_selection(
@@ -158,16 +219,17 @@ def _classify_selection(
     structure_altloc_mask: np.ndarray,
     structure_backbone_mask: np.ndarray,
     domain_shift_min_span: int,
-    loop_lddt_threshold: float,
+    scorer: SpanScorer,
+    loop_score_threshold: float,
 ) -> tuple[dict, set[tuple[str, int]]] | None:
     """Classify one contiguous altloc selection into a conformational type.
 
     1. If the span has no backbone altlocs anywhere, it is classified as ``side_chain_only``.
     2. Else if the longest contiguous backbone altloc run exceeds
        ``domain_shift_min_span``, it is classified as ``domain_shift``.
-    3. Else compute the per residue backbone lDDT for every altloc pair over
+    3. Else compute the per residue metric for every altloc pair over
        the backbone altloc residues in the span and take the minimum
-       pair mean. Compare against ``loop_lddt_threshold``, if it is above is is classified as
+       pair mean. Compare against ``loop_score_threshold``, if it is above is is classified as
        ``small_loop``. If it is below, it is classified as ``large_loop``.
 
     Returns ``(row_dict, covered_altloc_residues)`` on success or ``None`` if the
@@ -175,9 +237,9 @@ def _classify_selection(
 
     ``row_dict`` has the keys:
     ``protein``, ``selection``, ``chain``, ``start_res``, ``end_res``,
-    ``span_length``, ``classification``, ``worst_pair_mean_backbone_lddt``,
-    ``n_backbone_altloc_residues``, ``n_altlocs``, and ``pair_lddts`` (a
-    JSON encoded ``{pair_label: mean_lddt}`` map so the dict can be loaded
+    ``span_length``, ``classification``, ``score``, ``score_metric``,
+    ``n_backbone_altloc_residues``, ``n_altlocs``, and ``pair_scores`` (a
+    JSON encoded ``{pair_label: score}`` map so the dict can be loaded
     through the CSV intact via ``json.loads``).
 
     ``covered_altloc_residues`` is the set of ``(chain_id, res_id)`` pairs in the
@@ -243,8 +305,9 @@ def _classify_selection(
         "n_backbone_altloc_residues": n_backbone,
         "n_altlocs": len(altloc_ids),
         # JSON encoded so the pair calculation can be loaded back through the CSV
-        "pair_lddts": json.dumps({}),
-        "worst_pair_mean_backbone_lddt": float("nan"),
+        "pair_scores": json.dumps({}),
+        "score_metric": scorer.metric_name,
+        "score": float("nan"),
         "classification": "",
     }
 
@@ -258,28 +321,31 @@ def _classify_selection(
         row["classification"] = "domain_shift"
         return row, covered_altloc_residues
 
-    # Loop classification via pairwise lDDT across all altloc pairs
-    pair_lddts: dict[str, float] = {}
+    # Loop classification via pairwise strategy scores across all altloc pairs
+    pair_scores: dict[str, float] = {}
     for i in range(len(altloc_ids)):
         for j in range(i + 1, len(altloc_ids)):
             pair = pair_arrays.get((altloc_ids[i], altloc_ids[j]))
             gt, pred = pair if pair is not None else (None, None)
-            pair_lddts[f"{altloc_ids[i]}-{altloc_ids[j]}"] = _mean_residue_lddt_for_pair(
-                gt, pred, chain, backbone_altloc_res_ids
+            pair_scores[f"{altloc_ids[i]}-{altloc_ids[j]}"] = _score_pair_with_scorer(
+                scorer, gt, pred, chain, backbone_altloc_res_ids
             )
-    row["pair_lddts"] = json.dumps(pair_lddts)
+    row["pair_scores"] = json.dumps(pair_scores)
 
-    finite_vals = [v for v in pair_lddts.values() if np.isfinite(v)]
+    finite_vals = [v for v in pair_scores.values() if np.isfinite(v)]
     if not finite_vals:
         raise RuntimeError(
-            f"[{protein}] could not compute lDDT for any altloc pair in span "
-            f"'{selection_str}' (backbone-altloc residues: {backbone_altloc_res_ids}). "
+            f"[{protein}] could not compute {scorer.metric_name} for any altloc pair "
+            f"in span '{selection_str}' "
+            f"(backbone-altloc residues: {backbone_altloc_res_ids}). "
             "Refusing to emit an indeterminate classification."
         )
 
-    worst = float(min(finite_vals))
-    row["worst_pair_mean_backbone_lddt"] = worst
-    row["classification"] = "small_loop" if worst > loop_lddt_threshold else "large_loop"
+    worst = float(scorer.worse_pair_score(finite_vals))
+    row["score"] = worst
+    row["classification"] = (
+        "small_loop" if scorer.is_small_loop_score(worst, loop_score_threshold) else "large_loop"
+    )
     return row, covered_altloc_residues
 
 
@@ -287,7 +353,8 @@ def _process_structure(
     input_row: pd.Series,
     cif_root: Path | None,
     domain_shift_min_span: int,
-    loop_lddt_threshold: float,
+    scoring_strategy: SpanScorer,
+    loop_score_threshold: float,
 ) -> list[dict]:
     protein = str(input_row["protein"])
     cif_path = resolve_cif_path(input_row, cif_root)
@@ -314,15 +381,25 @@ def _process_structure(
     structure_altloc_mask = ~np.isin(atom_array.altloc_id, list(BLANK_ALTLOC_IDS))
     structure_backbone_mask = np.isin(atom_array.atom_name, BACKBONE_ATOM_TYPES)
 
+    # (chain, res_id) pairs that carry any altloc. Used to skip the redundant combined
+    # selection below and to check coverage after classification
+    all_altloc_res_ids: set[tuple[str, int]] = {
+        (str(c), int(r))
+        for c, r in zip(
+            atom_array.chain_id[structure_altloc_mask],
+            atom_array.res_id[structure_altloc_mask],
+        )
+    }
+
     rows: list[dict] = []
     classified_res_ids: set[tuple[str, int]] = set()
     for selection_str in [s.strip() for s in selection_field.split(";") if s.strip()]:
-        # find_altloc_selections.py appends a combined all altloc selection
-        # (atomworks-style with " or " clauses) at the end of each row. That one is
-        # a union over every span we already processed individually, so skip it.
-        # NOTE: This will need to be addressed when we
-        # migrate to atomworks-style selections for everything
-        if " or " in selection_str:
+        # find_altloc_selections.py may append a combined selection unioning every altloc
+        # span (atomworks-style "res_id == .. or .."). Skip only that redundant total union
+        # to avoid double-counting. True discontinuous "or" selections are still
+        # classified
+        covered_res_ids = selection_to_residues(atom_array, selection_str)
+        if covered_res_ids and covered_res_ids == all_altloc_res_ids:
             continue
         out = _classify_selection(
             atom_array=atom_array,
@@ -333,7 +410,8 @@ def _process_structure(
             structure_altloc_mask=structure_altloc_mask,
             structure_backbone_mask=structure_backbone_mask,
             domain_shift_min_span=domain_shift_min_span,
-            loop_lddt_threshold=loop_lddt_threshold,
+            scorer=scoring_strategy,
+            loop_score_threshold=loop_score_threshold,
         )
         if out is None:
             continue
@@ -343,13 +421,6 @@ def _process_structure(
 
     # residues across all classified spans should equal total unique
     # (chain, res_id) pairs that carry any altloc in the structure.
-    all_altloc_res_ids: set[tuple[str, int]] = {
-        (str(c), int(r))
-        for c, r in zip(
-            atom_array.chain_id[structure_altloc_mask],
-            atom_array.res_id[structure_altloc_mask],
-        )
-    }
     if classified_res_ids != all_altloc_res_ids:
         missing = all_altloc_res_ids - classified_res_ids
         extra = classified_res_ids - all_altloc_res_ids
@@ -364,6 +435,14 @@ def _process_structure(
 
 
 def main(args: argparse.Namespace) -> None:
+    """Run altloc region classification."""
+    scoring_strategy = LOOP_SCORERS[args.loop_score_metric]
+    loop_score_threshold = (
+        args.loop_score_threshold
+        if args.loop_score_threshold is not None
+        else scoring_strategy.default_threshold
+    )
+
     input_df = pd.read_csv(args.input_csv)
     required = {"protein", "selection"}
     missing = required - set(input_df.columns)
@@ -377,7 +456,8 @@ def main(args: argparse.Namespace) -> None:
                 input_row=row,
                 cif_root=args.cif_root,
                 domain_shift_min_span=args.domain_shift_min_span,
-                loop_lddt_threshold=args.loop_lddt_threshold,
+                scoring_strategy=scoring_strategy,
+                loop_score_threshold=loop_score_threshold,
             )
         )
 
@@ -411,6 +491,20 @@ def main(args: argparse.Namespace) -> None:
     )
     parser.add_argument("--output-file", type=Path, required=True)
     parser.add_argument("--domain-shift-min-span", type=int, default=50)
-    parser.add_argument("--loop-lddt-threshold", type=float, default=0.75)
+    parser.add_argument(
+        "--loop-score-metric",
+        choices=sorted(LOOP_SCORERS),
+        default="lddt",
+        help="Residue-level metric strategy used for small_loop / large_loop classification.",
+    )
+    parser.add_argument(
+        "--loop-score-threshold",
+        type=float,
+        default=None,
+        help=(
+            "Threshold for the selected loop scoring strategy. Defaults to the scorer specific"
+            "threshold when omitted."
+        ),
+    )
     args = parser.parse_args()
     main(args)