diff --git a/src/nmreval/models/diffusion.py b/src/nmreval/models/diffusion.py
index c479d06..fc492f7 100644
--- a/src/nmreval/models/diffusion.py
+++ b/src/nmreval/models/diffusion.py
@@ -111,7 +111,8 @@ class AnisotropicDiffusion(object):
         t = 2 * tp / 3 + tm
         # Callaghan eq (6.89)
         if HAS_C_FUNCS:
-            diffusion_decay = AnisotropicDiffusion._integrate_c(q, t, d_perp, d_par)
+            # divide by 2 to normalize by integral sin(x), x=0..pi
+            diffusion_decay = AnisotropicDiffusion._integrate_c(q, t, d_perp, d_par) / 2
         else:
             z = np.sqrt(q**2 * (d_par - d_perp) * t)
             diffusion_decay = np.exp(-q**2 * t * d_perp) * special.erf(z) / z