diff --git a/examples/api/custom_tmatrix/custom_tmatrices_simple.py b/examples/api/custom_tmatrix/custom_tmatrices_simple.py
new file mode 100755
index 0000000..b6f3d50
--- /dev/null
+++ b/examples/api/custom_tmatrix/custom_tmatrices_simple.py
@@ -0,0 +1,36 @@
+#!/usr/bin/env python3
+from qpms import TMatrixGenerator, BaseSpec, eV, hbar
+import numpy as np
+import sys
+
+errors = 0
+
+def tmg_diagonal_fun(tmatrix, omega):
+    '''
+    Example of a python function used as a custom T-matrix generator
+
+    It receives a CTMatrix argument with pre-filled BaseSpec
+    (in tmatrix.spec) and angular frequency.
+
+    It has to fill in the T-matrix elements tmatrix[...] 
+    (a numpy array of shape (len(tmatrix.spec),len(tmatrix.spec)))
+    and return zero (on success) or other integral value on error.
+
+    Note that this in justa an example of using the API,
+    not supposed to be anything physical.
+    '''
+    l = tmatrix.spec.l()
+    tmatrix[...] = np.diag(1./l**2)
+    return 0
+
+# Wrap the function as an actual TMatrixGenerator
+tmg_diagonal = TMatrixGenerator(tmg_diagonal_fun)
+
+bspec = BaseSpec(lMax=2)
+
+tmatrix = tmg_diagonal(bspec, (2.0+.01j) * eV/hbar)
+
+errors += np.sum(tmatrix[...] != np.diag(1./bspec.l()**2))
+
+sys.exit(errors)
+