diff --git a/pdme/util/fast_nonlocal_spectrum.py b/pdme/util/fast_nonlocal_spectrum.py
index 79998ff..a9f838d 100644
--- a/pdme/util/fast_nonlocal_spectrum.py
+++ b/pdme/util/fast_nonlocal_spectrum.py
@@ -98,3 +98,11 @@ def fast_s_nonlocal_dipoleses(
 
 	_logger.debug(f"Raw pair calc: [{alphses1 * alphses2 * bses}]")
 	return numpy.einsum("...j->...", alphses1 * alphses2 * bses)
+
+
+def signarg(x, **kwargs):
+	"""
+	uses numpy.sign to implement Arg for real numbers only. Should return pi for negative inputs, 0 for positive.
+	Passes through args to numpy.sign
+	"""
+	return numpy.pi * (numpy.sign(x, **kwargs) - 1) / (-2)
diff --git a/tests/util/__snapshots__/test_fast_nonlocal_spectrum.ambr b/tests/util/__snapshots__/test_fast_nonlocal_spectrum.ambr
new file mode 100644
index 0000000..82a208f
--- /dev/null
+++ b/tests/util/__snapshots__/test_fast_nonlocal_spectrum.ambr
@@ -0,0 +1,20 @@
+# serializer version: 1
+# name: test_arg
+  list([
+    list([
+      -0.0,
+      -0.0,
+      -0.0,
+    ]),
+    list([
+      3.141592653589793,
+      -0.0,
+      -0.0,
+    ]),
+    list([
+      -0.0,
+      -0.0,
+      3.141592653589793,
+    ]),
+  ])
+# ---
diff --git a/tests/util/test_fast_nonlocal_spectrum.py b/tests/util/test_fast_nonlocal_spectrum.py
index 49d6ae8..2f1a6c1 100644
--- a/tests/util/test_fast_nonlocal_spectrum.py
+++ b/tests/util/test_fast_nonlocal_spectrum.py
@@ -53,3 +53,11 @@ def test_fast_nonlocal_frequency_check():
 
 	with pytest.raises(ValueError):
 		pdme.util.fast_nonlocal_spectrum.fast_s_nonlocal(dot_pairs, dipoles)
+
+
+def test_arg(snapshot):
+
+	test_input = numpy.array([[1, 2, 3], [-1, 1, 3], [3, 5, -1]])
+
+	actual_result = pdme.util.fast_nonlocal_spectrum.signarg(test_input)
+	assert actual_result.tolist() == snapshot