diff --git a/CHANGELOG.md b/CHANGELOG.md
index 854a7df..832b385 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -14,6 +14,11 @@ and this project follows [Semantic Versioning](https://semver.org/spec/v2.0.0.ht
   images and JupyterHub profiles can omit it (default off); instructors enable
   cluster testing with `QUANTUI_ENABLE_SLURM=1` without any extra student
   command.
+- **CPU teaching SIF defaults** — `quantui.def` sets `QUANTUI_FREQ_PARALLEL=1`
+  so NCShare classroom sessions parallelize IR-intensity displacements on
+  multi-core nodes. The Settings checkbox reflects the deployment value and is
+  locked while the variable is set; use `QUANTUI_FREQ_PARALLEL=0` to force off
+  site-wide.
 
 ### Fixed
 
diff --git a/apptainer/README.md b/apptainer/README.md
index 276941b..1a98809 100644
--- a/apptainer/README.md
+++ b/apptainer/README.md
@@ -35,6 +35,13 @@ every layer (CUDA base image, all-pip install, a pinned PyPI release rather than
 the working tree). Merging them would ship a multi-GB CUDA stack to students who
 will never have a GPU. See [GPU image](#gpu-image) below.
 
+**CPU image environment defaults** (set in `quantui.def` `%environment`):
+
+| Variable | CPU `quantui.sif` | GPU `quantui-gpu.sif` |
+| --- | --- | --- |
+| `QUANTUI_FREQ_PARALLEL` | `1` — parallel CPU workers for IR-intensity displacements on frequency calcs (≥4 cores, ≥2 atoms). Overrides the Settings checkbox while set. | unset (off unless you set it) |
+| `QUANTUI_ENABLE_SLURM` | unset (class stays in-kernel) | unset |
+
 The compiled `.sif` image is **not** committed to git — it is too large (~4–5 GB).
 Build it locally (see below) or download the latest release asset from the
 [GitHub Releases page](https://github.com/The-Schultz-Lab/QuantUI/releases).
diff --git a/apptainer/quantui.def b/apptainer/quantui.def
index cffa3be..17f7339 100644
--- a/apptainer/quantui.def
+++ b/apptainer/quantui.def
@@ -49,6 +49,12 @@ FROM: condaforge/miniforge3:26.1.1-3
   # across kernel restarts. The home dir is bind-mounted by Apptainer.
   export QUANTUI_RESULTS_DIR="${HOME}/.quantui/results"
 
+  # Parallel CPU workers for IR-intensity finite-difference SCFs (6N displacements)
+  # on multi-core hosts. Reference SCF + Hessian stay in-kernel. Students can still
+  # see the preference in Settings; when this variable is set it overrides the
+  # saved checkbox (set QUANTUI_FREQ_PARALLEL=0 to force off site-wide).
+  export QUANTUI_FREQ_PARALLEL=1
+
 %files
   # Source paths are relative to where you run `apptainer build` (repo root).
   # Run the build command from the repo root, not from apptainer/.
diff --git a/quantui/app.py b/quantui/app.py
index 0c03e0f..008f56c 100644
--- a/quantui/app.py
+++ b/quantui/app.py
@@ -574,6 +574,10 @@
     SUPPORTED_BASIS_SETS,
     SUPPORTED_METHODS,
 )
+from quantui.freq_ir_workers import (
+    freq_parallel_env_configured,
+    freq_parallel_opt_in,
+)
 from quantui.help_content import HELP_TOPICS
 from quantui.molecule import Molecule, parse_xyz_input
 from quantui.progress import StepProgress
@@ -1964,7 +1968,12 @@ def _build_status_panel(self) -> None:
             vib_framerate_fps=self._user_settings.viz.vib_framerate_fps,
             gpu_enabled=self._user_settings.compute.gpu_enabled,
             density_fit_enabled=self._user_settings.compute.density_fit,
-            freq_parallel_enabled=self._user_settings.compute.freq_parallel,
+            freq_parallel_enabled=(
+                freq_parallel_opt_in()
+                if freq_parallel_env_configured()
+                else self._user_settings.compute.freq_parallel
+            ),
+            freq_parallel_env_locked=freq_parallel_env_configured(),
             execution_backend=self._user_settings.compute.execution_backend,
             slurm_available=is_slurm_available(),
         )
@@ -3748,10 +3757,12 @@ def _on_density_fit_enabled_changed(self, change) -> None:
     def _on_freq_parallel_enabled_changed(self, change) -> None:
         """Persist the parallel IR finite-difference preference.
 
-        The freq_calc driver reads this via :func:`freq_ir_workers._freq_parallel_opt_in`
+        The freq_calc driver reads this via :func:`freq_ir_workers.freq_parallel_opt_in`
         on each run (unless ``QUANTUI_FREQ_PARALLEL`` overrides in the environment).
         Refresh the time estimate because parallel divides the IR term in the model.
         """
+        if freq_parallel_env_configured():
+            return
         new_val = bool(change["new"])
         if new_val == self._user_settings.compute.freq_parallel:
             return
diff --git a/quantui/app_builders.py b/quantui/app_builders.py
index f66fdb5..31876f1 100644
--- a/quantui/app_builders.py
+++ b/quantui/app_builders.py
@@ -123,6 +123,7 @@ def build_status_panel(
     gpu_enabled: bool = True,
     density_fit_enabled: bool = False,
     freq_parallel_enabled: bool = False,
+    freq_parallel_env_locked: bool = False,
     execution_backend: str = "local",
     slurm_available: bool = False,
 ) -> None:
@@ -327,8 +328,17 @@ def _render_status(gpu_state: Any) -> str:
         value=freq_parallel_enabled,
         description="Parallelize IR intensity displacements (CPU)",
         indent=False,
+        disabled=freq_parallel_env_locked,
         layout=layout_fn(width="320px", margin="2px 0 0 0"),
     )
+    if freq_parallel_env_locked:
+        fp_env_note = widgets.HTML(
+            f'<div style="font-size:11px;color:{_theme.css.TEXT_SUBTLE};margin:2px 0 0 0">'
+            "Controlled by deployment (<code>QUANTUI_FREQ_PARALLEL</code> in the "
+            "container or session environment).</div>"
+        )
+    else:
+        fp_env_note = None
 
     exec_backend_label = widgets.HTML(
         f'<div style="font-size:12px;color:{_theme.css.TEXT_SLATE_DARK};margin-top:12px;'
@@ -360,6 +370,13 @@ def _render_status(gpu_state: Any) -> str:
             "Use the Cluster Jobs tab to monitor and cancel runs.</div>"
         )
 
+    fp_settings_rows: list[Any] = [
+        fp_toggle_label,
+        app.freq_parallel_enabled_cb,
+    ]
+    if fp_env_note is not None:
+        fp_settings_rows.append(fp_env_note)
+
     settings_box = widgets.VBox(
         [
             settings_html,
@@ -370,8 +387,7 @@ def _render_status(gpu_state: Any) -> str:
             app.gpu_enabled_cb,
             df_toggle_label,
             app.density_fit_enabled_cb,
-            fp_toggle_label,
-            app.freq_parallel_enabled_cb,
+            *fp_settings_rows,
             exec_backend_label,
             app.execution_backend_dd,
             exec_backend_note,
diff --git a/quantui/freq_ir_workers.py b/quantui/freq_ir_workers.py
index 9e29fcf..cbc2fbe 100644
--- a/quantui/freq_ir_workers.py
+++ b/quantui/freq_ir_workers.py
@@ -169,6 +169,16 @@ def run_displaced_scf(coords_bohr_flat) -> Any:
     return np.array(mf.dip_moment(verbose=0))
 
 
+def freq_parallel_opt_in() -> bool:
+    """Return whether parallel IR displacements are enabled for the next run."""
+    return _freq_parallel_opt_in()
+
+
+def freq_parallel_env_configured() -> bool:
+    """Return True when ``QUANTUI_FREQ_PARALLEL`` is set in the environment."""
+    return os.environ.get("QUANTUI_FREQ_PARALLEL") is not None
+
+
 def _freq_parallel_opt_in() -> bool:
     """Whether parallel IR displacements are opted in.
 
diff --git a/tests/test_cpu_container_def.py b/tests/test_cpu_container_def.py
new file mode 100644
index 0000000..ce7becb
--- /dev/null
+++ b/tests/test_cpu_container_def.py
@@ -0,0 +1,39 @@
+"""CPU Apptainer image deployment defaults (quantui.def).
+
+Lightweight text assertions — no Apptainer build required.
+"""
+
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+import pytest
+
+REPO = Path(__file__).resolve().parent.parent
+CPU_DEF = REPO / "apptainer" / "quantui.def"
+GPU_DEF = REPO / "apptainer" / "quantui-gpu.def"
+
+
+@pytest.fixture(scope="module")
+def cpu_def_text() -> str:
+    return CPU_DEF.read_text(encoding="utf-8")
+
+
+def _section(text: str, name: str) -> str:
+    m = re.search(rf"^{re.escape(name)}\s*$", text, re.M)
+    assert m is not None, f"no {name} section"
+    rest = text[m.end() :]
+    nxt = re.search(r"^%[a-z]+\s*$", rest, re.M)
+    return rest[: nxt.start()] if nxt else rest
+
+
+class TestCpuTeachingImageDefaults:
+    def test_cpu_image_enables_ir_parallel_by_default(self, cpu_def_text):
+        env = _section(cpu_def_text, "%environment")
+        assert re.search(r"export\s+QUANTUI_FREQ_PARALLEL=1\b", env)
+
+    def test_gpu_image_does_not_inherit_cpu_ir_parallel_default(self):
+        gpu_text = GPU_DEF.read_text(encoding="utf-8")
+        env = _section(gpu_text, "%environment")
+        assert "QUANTUI_FREQ_PARALLEL" not in env
diff --git a/tests/test_freq_ir_workers.py b/tests/test_freq_ir_workers.py
index 29d5326..5aa55e0 100644
--- a/tests/test_freq_ir_workers.py
+++ b/tests/test_freq_ir_workers.py
@@ -17,6 +17,8 @@
 
 from quantui.freq_ir_workers import (
     _truthy,
+    freq_parallel_env_configured,
+    freq_parallel_opt_in,
     parallel_enabled_for_run,
     pick_worker_count,
     threads_per_worker,
@@ -138,3 +140,15 @@ def test_recognised_falsy(self, value):
     def test_whitespace_stripped(self):
         assert _truthy("  1  ") is True
         assert _truthy("\ttrue\n") is True
+
+
+class TestEnvConfiguredHelpers:
+    def test_freq_parallel_env_configured(self, monkeypatch):
+        monkeypatch.delenv("QUANTUI_FREQ_PARALLEL", raising=False)
+        assert freq_parallel_env_configured() is False
+        monkeypatch.setenv("QUANTUI_FREQ_PARALLEL", "0")
+        assert freq_parallel_env_configured() is True
+
+    def test_freq_parallel_opt_in_public_alias(self, monkeypatch):
+        monkeypatch.setenv("QUANTUI_FREQ_PARALLEL", "yes")
+        assert freq_parallel_opt_in() is True