diff --git a/pqsetup/input_writer.py b/pqsetup/input_writer.py
index adf12a6..4276ed5 100644
--- a/pqsetup/input_writer.py
+++ b/pqsetup/input_writer.py
@@ -256,6 +256,16 @@ def validate_setup(
                 ),
             )
         )
+    if setup.job_type == "mm-opt" and setup.ensemble != "OPT":
+        diagnostics.append(
+            _error(
+                "workflow.mm_opt_ensemble",
+                (
+                    "mm-opt is an optimization job type and cannot use an "
+                    "MD ensemble."
+                ),
+            )
+        )
     if setup.job_type == "qm-rpmd":
         beads = setup.extra_settings.get("rpmd_n_replica")
         if beads is None:
diff --git a/tests/test_presets_and_input.py b/tests/test_presets_and_input.py
index afa3a9c..248c90d 100644
--- a/tests/test_presets_and_input.py
+++ b/tests/test_presets_and_input.py
@@ -73,6 +73,18 @@ def test_incomplete_mm_optimization_package_is_rejected() -> None:
     assert {item.code for item in result.diagnostics} == {"workflow.unsupported"}
 
 
+def test_mm_opt_with_md_ensemble_is_rejected() -> None:
+    result = render_input(
+        SimulationSetup(
+            job_type="mm-opt",
+            ensemble="NVT",
+            thermostat="berendsen",
+            runner=None,
+        )
+    )
+
+    assert not result.valid
+    assert "workflow.mm_opt_ensemble" in {item.code for item in result.diagnostics}
 def test_qm_rpmd_requires_bead_count() -> None:
     missing = render_input(
         SimulationSetup(