diff --git a/src/diffusion/loss.py b/src/diffusion/loss.py
index d48db47..02fa261 100644
--- a/src/diffusion/loss.py
+++ b/src/diffusion/loss.py
@@ -1,8 +1,8 @@
 """Diffusion losses"""
 
 import torch
-from src.utils.geometry.vector import Vec3Array, square_euclidean_distance
-from torch import nn
+from src.utils.geometry.vector import Vec3Array, square_euclidean_distance, euclidean_distance
+from torch.nn import functional as F
 
 
 def smooth_lddt_loss(
@@ -13,7 +13,32 @@ def smooth_lddt_loss(
         mask: torch.Tensor = None  # (bs, n_atoms)
 ) -> torch.Tensor:  # (bs,)
     """Smooth local distance difference test (LDDT) auxiliary loss."""
-    raise NotImplementedError
+    bs, n_atoms = pred_atoms.shape
+
+    # Compute distances between all pairs of atoms
+    delta_x_lm = euclidean_distance(pred_atoms[:, :, None], pred_atoms[:, None, :])  # (bs, n_atoms, n_atoms)
+    delta_x_gt_lm = euclidean_distance(gt_atoms[:, :, None], gt_atoms[:, None, :])
+
+    # Compute distance difference for all pairs of atoms
+    delta_lm = torch.abs(delta_x_gt_lm - delta_x_lm)  # (bs, n_atoms, n_atoms)
+    epsilon_lm = torch.div((F.sigmoid(torch.sub(0.5, delta_lm)) + F.sigmoid(torch.sub(1.0, delta_lm)) +
+                            F.sigmoid(torch.sub(2.0, delta_lm)) + F.sigmoid(torch.sub(4.0, delta_lm))), 4.0)
+
+    # Restrict to bespoke inclusion radius
+    atom_is_nucleotide = atom_is_dna + atom_is_rna
+    atom_not_nucleotide = torch.add(torch.neg(atom_is_nucleotide), 1.0)  # (1 - atom_is_nucleotide)
+    c_lm = (delta_x_gt_lm < 30.0).float() * atom_is_nucleotide + (delta_x_gt_lm < 15.0).float() * atom_not_nucleotide
+
+    # Mask positions
+    if mask is not None:
+        c_lm *= (mask[:, :, None] * mask[:, None, :])
+
+    # Compute mean, avoiding self-term
+    self_mask = torch.eye(n_atoms).unsqueeze(0).expand_as(c_lm).to(c_lm.device)  # (bs, n_atoms, n_atoms)
+    self_mask = torch.add(torch.neg(self_mask), 1.0)
+    c_lm *= self_mask
+    lddt = torch.mean(epsilon_lm * c_lm, dim=(1, 2)) / torch.mean(c_lm, dim=(1, 2))
+    return torch.add(torch.neg(lddt), 1.0)  # (1 - lddt)
 
 
 def mean_squared_error(
diff --git a/tests/test_loss.py b/tests/test_loss.py
index 29bb43e..d124156 100644
--- a/tests/test_loss.py
+++ b/tests/test_loss.py
@@ -1,6 +1,6 @@
 import unittest
 import torch
-from src.diffusion.loss import mean_squared_error
+from src.diffusion.loss import mean_squared_error, smooth_lddt_loss
 from src.utils.geometry.vector import Vec3Array
 
 
@@ -44,5 +44,50 @@ def test_mse_with_mask(self):
         self.assertTrue(torch.allclose(mse, expected_mse), f"Expected MSE {expected_mse}, got {mse}")
 
 
+class TestSmoothLDDTLoss(unittest.TestCase):
+
+    def test_basic_functionality(self):
+        # Create a simple scenario where pred_atoms and gt_atoms are the same, no nucleotide-specific settings.
+        bs, n_atoms = 1, 4
+        pred_atoms = Vec3Array.from_array(torch.tensor([[[0., 0., 0.], [1., 0., 0.], [2., 0., 0.], [3., 0., 0.]]]))
+        gt_atoms = Vec3Array.from_array(torch.tensor([[[0., 0., 0.], [1., 0., 0.], [2., 0., 0.], [3., 0., 0.]]]))
+        atom_is_rna = torch.zeros((bs, n_atoms))
+        atom_is_dna = torch.zeros((bs, n_atoms))
+        mask = None
+
+        # Expected loss is 0 since pred_atoms == gt_atoms
+        loss_when_identical = smooth_lddt_loss(pred_atoms, gt_atoms, atom_is_rna, atom_is_dna, mask)
+
+        # Add noise and compare
+        noisy_pred_atoms = pred_atoms + 0.1 * Vec3Array.from_array(torch.randn((bs, n_atoms, 3)))
+        noisier_pred_atoms = pred_atoms + 1.0 * Vec3Array.from_array(torch.randn((bs, n_atoms, 3)))
+
+        loss_when_noisy = smooth_lddt_loss(noisy_pred_atoms, gt_atoms, atom_is_rna, atom_is_dna, mask)
+        loss_when_noisier = smooth_lddt_loss(noisier_pred_atoms, gt_atoms, atom_is_rna, atom_is_dna, mask)
+
+        self.assertTrue(torch.all((loss_when_identical < loss_when_noisy) & (loss_when_noisy < loss_when_noisier)))
+
+    def test_mask(self):
+        # Create a simple scenario where pred_atoms and gt_atoms are the same, no nucleotide-specific settings.
+        bs, n_atoms = 1, 4
+        pred_atoms = Vec3Array.from_array(torch.tensor([[[0., 0., 0.], [1., 0., 0.], [2., 0., 0.], [3., 0., 0.]]]))
+        gt_atoms = Vec3Array.from_array(torch.tensor([[[0., 0., 0.], [1., 0., 0.], [2., 0., 0.], [3., 0., 0.]]]))
+        atom_is_rna = torch.randint(0, 2, (bs, n_atoms))
+        atom_is_dna = torch.zeros((bs, n_atoms))
+        mask = torch.ones((bs, n_atoms))
+
+        # Expected loss is 0 since pred_atoms == gt_atoms
+        loss_when_identical = smooth_lddt_loss(pred_atoms, gt_atoms, atom_is_rna, atom_is_dna, mask)
+
+        # Add noise and compare
+        noisy_pred_atoms = pred_atoms + 5.0 * Vec3Array.from_array(torch.randn((bs, n_atoms, 3)))
+        noisier_pred_atoms = pred_atoms + 10.0 * Vec3Array.from_array(torch.randn((bs, n_atoms, 3)))
+
+        loss_when_noisy = smooth_lddt_loss(noisy_pred_atoms, gt_atoms, atom_is_rna, atom_is_dna, mask)
+        loss_when_noisier = smooth_lddt_loss(noisier_pred_atoms, gt_atoms, atom_is_rna, atom_is_dna, mask)
+
+        self.assertTrue(torch.all((loss_when_identical < loss_when_noisy) & (loss_when_noisy < loss_when_noisier)))
+
+
 if __name__ == '__main__':
     unittest.main()