diff --git a/image_analysis/structure_refinement/ModelValidation.cpp b/image_analysis/structure_refinement/ModelValidation.cpp
index 3673483dc..3bbf9af2d 100644
--- a/image_analysis/structure_refinement/ModelValidation.cpp
+++ b/image_analysis/structure_refinement/ModelValidation.cpp
@@ -32,6 +32,7 @@
 #include "../../common/CorrelationCoefficient.h"
 #include "../../common/JFJochMath.h"   // PI (M_PI is not standard, and MSVC does not define it)
 #include "../../common/Logger.h"
+#include "../../common/ParallelFor.h"
 #include "../scale_merge/ReindexAmbiguity.h"   // ReindexReflections
 #include "../scale_merge/CrystalSetting.h"     // CellMappingOperators
 #include "ModelFFT.h"
@@ -481,13 +482,15 @@ ModelValidationResult Validate(const std::vector<MergedReflection> &merged,
                         "is not how the data describe this lattice; scoring {} change(s) of basis",
                         st.cell.a, st.cell.b, st.cell.c, st.cell.alpha, st.cell.beta, st.cell.gamma,
                         sg->xhm(), frames.size() - 1);
-            size_t best = 0;
-            double best_r = 0;
-            for (size_t i = 0; i < frames.size(); i++) {
+            // The candidates are scored at the same time, each on its own copy of the model, and read
+            // in their own order below, so the ranking and its tie-break are those of one after another.
+            std::vector<double> frame_r(frames.size(), 0.0);
+            std::vector<const gemmi::SpaceGroup *> frame_sg(frames.size(), nullptr);
+            ParallelFor(static_cast<int>(frames.size()), nthreads, [&](int i) {
                 gemmi::Structure trial = st;
                 const gemmi::SpaceGroup *trial_sg = sg;
                 if (i > 0 && !change_model_basis(trial, trial_sg, frames[i]))
-                    continue;   // no origin shift names the transformed group; not a basis we can take
+                    return;   // no origin shift names the transformed group; not a basis we can take
                 refractionalize_into(trial, data_cell);
                 trial.setup_cell_images();
                 // Each frame at the data's best indexing: the alternative indexings are only probed
@@ -499,8 +502,17 @@ ModelValidationResult Validate(const std::vector<MergedReflection> &merged,
                 if (same_point_group)
                     for (const gemmi::Op &law : reindex_ops)
                         r = std::min(r, frame_probe_r(trial, trial_sg, ReindexReflections(obs, law), d_min));
+                frame_r[i] = r;
+                frame_sg[i] = trial_sg;
+            });
+            size_t best = 0;
+            double best_r = 0;
+            for (size_t i = 0; i < frames.size(); i++) {
+                if (frame_sg[i] == nullptr)
+                    continue;
+                const double r = frame_r[i];
                 logger.Info("Model validation:   {:<12} -> {:<12} R {:.4f} on the coarse shell",
-                            frames[i].triplet(), trial_sg->xhm(), r);
+                            frames[i].triplet(), frame_sg[i]->xhm(), r);
                 if (i == 0 || r < best_r) {
                     best_r = r;
                     best = i;
diff --git a/image_analysis/structure_refinement/ModelValidation.h b/image_analysis/structure_refinement/ModelValidation.h
index 47bffdd26..bcfcad445 100644
--- a/image_analysis/structure_refinement/ModelValidation.h
+++ b/image_analysis/structure_refinement/ModelValidation.h
@@ -219,8 +219,8 @@ struct ModelValidationResult {
 // the runner-up beats the lead the same model in a random orientation takes, since a random model
 // also picks a winner. Holohedral crystals (no twin laws) are unaffected either way.
 //
-// nthreads is what the null's replicates run on - they are independent of each other and of the real
-// model, so they run at once. Nothing else here is threaded, and the answer does not depend on it.
+// nthreads is what the null's replicates and the candidate changes of basis run on - each is
+// independent of the others, so they run at once. The answer does not depend on it.
 //
 // report_shell_d_min is the merge statistics' own shell bounds, coarse to fine, and it is what
 // cc_model_shells is binned on. Sharing the grid is the point: a reader has to be able to put a