fix(qe): use ndmin=2 in np.loadtxt to handle single-line .evp files

dpdata/qe/traj.py

@@ -169,7 +169,7 @@ def load_data(fname: FileType, natoms, begin=0, step=1, convert=1.0):
 
 
 def load_energy(fname, begin=0, step=1):
-    data = np.loadtxt(fname)
+    data = np.loadtxt(fname, ndmin=2)
     steps = []
     for ii in data[begin::step, 0]:
         steps.append("%d" % ii)  # noqa: UP031

tests/test_qe_single_line_evp.py

@@ -0,0 +1,86 @@
+from __future__ import annotations
+
+import os
+import tempfile
+import unittest
+
+import dpdata
+
+
+class TestQESingleLineEvp(unittest.TestCase):
+    """Test for QE CP trajectory with single-line .evp file (issue #899)."""
+
+    def setUp(self):
+        # Create a temporary directory for test files
+        self.test_dir = tempfile.mkdtemp()
+
+        # Create a simple input file
+        self.input_content = """&CONTROL
+  calculation = 'cp'
+  prefix = 'test'
+/
+&SYSTEM
+  ibrav = 1
+  celldm(1) = 10.0
+  nat = 2
+  ntyp = 1
+/
+&ELECTRONS
+/
+&IONS
+/
+ATOMIC_SPECIES
+H 1.0 H.pbe-rrkjus_psl.1.0.0.UPF
+ATOMIC_POSITIONS {alat}
+H 0.0 0.0 0.0
+H 0.5 0.5 0.5
+"""
+
+        # Create a single-line .evp file (this used to cause errors)
+        self.evp_content = "# comments here\n    195  9.433649E-03  1.290500E-05  0.000000E+00  1.057182E-02      -1100.03076389      -1100.03076389      -1100.03075434      -1100.03073209  1.300606E+04      -3.31610"
+
+        # Create a single-line .for file
+        self.for_content = "    195\n0.0 0.0 0.0\n0.0 0.0 0.0"
+
+        # Create a single-line .pos file
+        self.pos_content = "    195\n0.0 0.0 0.0\n1.0 1.0 1.0"
+
+        # Create a .cel file
+        self.cel_content = "    195\n10.0 0.0 0.0\n0.0 10.0 0.0\n0.0 0.0 10.0"
+
+        # Write files
+        self.prefix = os.path.join(self.test_dir, "test")
+        with open(self.prefix + ".in", "w") as f:
+            f.write(self.input_content)
+        with open(self.prefix + ".evp", "w") as f:
+            f.write(self.evp_content)
+        with open(self.prefix + ".for", "w") as f:
+            f.write(self.for_content)
+        with open(self.prefix + ".pos", "w") as f:
+            f.write(self.pos_content)
+        with open(self.prefix + ".cel", "w") as f:
+            f.write(self.cel_content)
+
+    def tearDown(self):
+        # Clean up temporary files
+        import shutil
+
+        shutil.rmtree(self.test_dir)
+
+    def test_single_line_evp_load(self):
+        """Test that single-line .evp files can be loaded without errors."""
+        # This should not raise an exception with the fix
+        system = dpdata.LabeledSystem(self.prefix, fmt="qe/cp/traj")
+
+        # Verify basic properties
+        self.assertEqual(system.get_nframes(), 1)
+        self.assertEqual(len(system["energies"]), 1)
+
+        # The energy should be converted properly
+        # -1100.03076389 Hartree * energy_convert factor
+        # Let's check the actual value first and use a more reasonable tolerance
+        self.assertAlmostEqual(system["energies"][0], -29933.361998680055, places=3)
+
+
+if __name__ == "__main__":
+    unittest.main()