diff --git a/PQAnalysis/io/nep/nep_writer.py b/PQAnalysis/io/nep/nep_writer.py
index 4289054c..baf9a856 100644
--- a/PQAnalysis/io/nep/nep_writer.py
+++ b/PQAnalysis/io/nep/nep_writer.py
@@ -467,10 +467,14 @@ def _setup_frame_splitting_for_training(
                     exception=NEPError
                 )
 
-        sum_frames = n_train + n_test + n_validation
+        if total_ratios is not None:
+            sum_frames = n_train + n_test + n_validation
 
-        self.test_ratio = n_test / sum_frames
-        self.validation_ratio = n_validation / sum_frames
+            self.test_ratio = n_test / sum_frames
+            self.validation_ratio = n_validation / sum_frames
+        else:
+            self.test_ratio = float(test_ratio)
+            self.validation_ratio = 0.0
 
         if self.test_ratio > 1.0:
             self.logger.error(
@@ -816,7 +820,7 @@ def write_from_trajectory(
         self.open()
         for frame in trajectory:
             self.write_from_atomic_system(
-                frame, use_forces, use_stress, use_virial
+                frame, self.file, use_forces, use_stress, use_virial
             )
 
         self.close()
diff --git a/tests/io/test_nepWriter.py b/tests/io/test_nepWriter.py
new file mode 100644
index 00000000..7a16297e
--- /dev/null
+++ b/tests/io/test_nepWriter.py
@@ -0,0 +1,103 @@
+import os
+
+import numpy as np
+
+from . import pytestmark
+
+from PQAnalysis.io.nep.nep_writer import NEPWriter
+from PQAnalysis.traj import Trajectory
+from PQAnalysis.core import Atom, Cell
+from PQAnalysis.atomic_system import AtomicSystem
+
+INFO_FILE_CONTENT = """\
+-----------------------------------------------------------------------------------------
+|                                  PQ info file                                  |
+-----------------------------------------------------------------------------------------
+|   SIMULATION-TIME         5.00000 ps       TEMPERATURE           294.45308 K          |
+|   PRESSURE             2875.71714 bar      E(TOT)            -186008.40141 kcal/mol   |
+|   E(QM)             -186197.10854 kcal/mol N(QM-ATOMS)             0.00000 -          |
+|   E(KIN)                188.70714 kcal/mol E(INTRA)                0.00000 kcal/mol   |
+|   VOLUME               4310.41014 A^3      DENSITY                 0.83455 g/cm^3     |
+|   MOMENTUM                2.5e-10 amuA/fs  LOOPTIME                0.90443 s          |
+-----------------------------------------------------------------------------------------
+"""
+
+
+
+class TestNEPWriter:
+
+    def test_write_from_files_with_test_ratio(self, tmpdir):
+        xyz_frame = (
+            "2 10.0 10.0 10.0 90.0 90.0 90.0\n#\n"
+            "C 0.0 0.0 0.0\nH 1.0 1.0 1.0\n"
+        )
+        with open("md.xyz", "w", encoding="utf-8") as file:
+            file.write(xyz_frame * 4)
+
+        with open("md.info", "w", encoding="utf-8") as file:
+            file.write(INFO_FILE_CONTENT)
+
+        with open("md.instant_en", "w", encoding="utf-8") as file:
+            for i in range(1, 5):
+                file.write(
+                    f"{i} 298.0 23000.0 -185898.0 -18590{i}.0 0.0 5.6 0.0 "
+                    "3729.1 0.83 1.1e-16 1.0\n"
+                )
+
+        writer = NEPWriter("out.xyz")
+        writer.write_from_files(["md"], test_ratio=0.25)
+
+        assert np.isclose(writer.test_ratio, 0.25)
+        assert np.isclose(writer.validation_ratio, 0.0)
+        assert writer.n_train_frames == 3
+        assert writer.n_test_frames == 1
+        assert os.path.exists("out.xyz_train")
+        assert os.path.exists("out.xyz_test")
+
+    def test_write_from_files_with_total_ratios(self, tmpdir):
+        xyz_frame = (
+            "2 10.0 10.0 10.0 90.0 90.0 90.0\n#\n"
+            "C 0.0 0.0 0.0\nH 1.0 1.0 1.0\n"
+        )
+        with open("md.xyz", "w", encoding="utf-8") as file:
+            file.write(xyz_frame * 4)
+
+        with open("md.info", "w", encoding="utf-8") as file:
+            file.write(INFO_FILE_CONTENT)
+
+        with open("md.instant_en", "w", encoding="utf-8") as file:
+            for i in range(1, 5):
+                file.write(
+                    f"{i} 298.0 23000.0 -185898.0 -18590{i}.0 0.0 5.6 0.0 "
+                    "3729.1 0.83 1.1e-16 1.0\n"
+                )
+
+        writer = NEPWriter("out.xyz")
+        writer.write_from_files(["md"], total_ratios="3:1")
+
+        assert np.isclose(writer.test_ratio, 0.25)
+        assert np.isclose(writer.validation_ratio, 0.0)
+        assert writer.n_train_frames == 3
+        assert writer.n_test_frames == 1
+
+    def test_write_from_trajectory(self, tmpdir):
+        system = AtomicSystem(
+            atoms=[Atom("C"), Atom("H")],
+            pos=np.array([[0.0, 0.0, 0.0], [1.0, 1.0, 1.0]]),
+            cell=Cell(10.0, 10.0, 10.0),
+        )
+        system.energy = -1.5
+        trajectory = Trajectory([system, system])
+
+        writer = NEPWriter("traj_out.xyz")
+        writer.write_from_trajectory(trajectory)
+
+        with open("traj_out.xyz", "r", encoding="utf-8") as file:
+            lines = file.read().splitlines()
+
+        assert len(lines) == 8
+        assert lines[0] == "2"
+        assert lines[4] == "2"
+        assert lines[1].startswith("energy=")
+        assert lines[2].startswith("C")
+        assert lines[3].startswith("H")