fix(abacus): omit unavailable MD force labels (deepmodeling#1024)

Fixes deepmodeling#1000.

Represent missing ABACUS MD FORCE columns as unavailable labels instead
of fabricated zero arrays.

Tests: `cd tests && python -m unittest test_abacus_md.TestABACUSMD`

Why existing tests missed it: All existing ABACUS MD fixtures contained
FORCE columns; neither the helper nor full LabeledSystem path covered a
no-force dump.

Coding agent: Codex
Codex version: codex-cli 0.144.4
Model: gpt-5.6-sol
Reasoning effort: xhigh

Co-authored-by: njzjz-bot <njzjz.bot@gmail.com>

Author: njzjz-bot

dpdata/formats/abacus/md.py

@@ -72,7 +72,10 @@ def get_coords_from_dump(dumplines, natoms):
     )
     cells = np.zeros([nframes_dump, 3, 3])
     stresses = np.zeros([nframes_dump, 3, 3])
-    forces = np.zeros([nframes_dump, total_natoms, 3])
+    # A missing FORCE column means labels are unavailable, not that every
+    # force is exactly zero.  Keep this distinction so callers never train on
+    # fabricated labels that merely look well-shaped.
+    forces = np.zeros([nframes_dump, total_natoms, 3]) if calc_force else None
     coords = np.zeros([nframes_dump, total_natoms, 3])
     iframe = 0
     for iline in range(nlines):
@@ -116,6 +119,7 @@ def get_coords_from_dump(dumplines, natoms):
                     coords[iframe, iat] *= celldm
 
                 if calc_force:
+                    assert forces is not None
                     forces[iframe, iat] = np.array(
                         [
                             float(i)
@@ -187,7 +191,8 @@ def get_frame(fname):
         if np.isnan(iene):
             coords = np.delete(coords, i - ndump, axis=0)
             cells = np.delete(cells, i - ndump, axis=0)
-            force = np.delete(force, i - ndump, axis=0)
+            if force is not None:
+                force = np.delete(force, i - ndump, axis=0)
             stress = np.delete(stress, i - ndump, axis=0)
             energy = np.delete(energy, i - ndump, axis=0)
             unconv_stru += "%d " % i  # noqa: UP031
@@ -207,7 +212,8 @@ def get_frame(fname):
     #    data['cells'][:, :, :] = cell
     data["coords"] = coords
     data["energies"] = energy
-    data["forces"] = force
+    if force is not None:
+        data["forces"] = force
     data["virials"] = stress
     if not isinstance(data["virials"], np.ndarray):
         del data["virials"]

tests/test_abacus_md.py

@@ -1,11 +1,14 @@
 from __future__ import annotations
 
 import os
+import shutil
+import tempfile
 import unittest
 
 import numpy as np
 from context import dpdata
 
+from dpdata.formats.abacus.md import get_coords_from_dump
 from dpdata.unit import LengthConversion
 
 bohr2ang = LengthConversion("bohr", "angstrom").value()
@@ -30,6 +33,47 @@ def tearDown(self):
         if os.path.isfile("abacus.md/water_stru"):
             os.remove("abacus.md/water_stru")
 
+    def test_missing_force_column_does_not_create_zero_labels(self):
+        dump = """MDSTEP: 0
+LATTICE_CONSTANT: 1.0 Angstrom
+LATTICE_VECTORS
+  1.0 0.0 0.0
+  0.0 1.0 0.0
+  0.0 0.0 1.0
+POSITION
+  0 H 0.0 0.0 0.0
+
+
+""".splitlines()
+        _, _, forces, _ = get_coords_from_dump(dump, [1])
+        self.assertIsNone(forces)
+
+    def test_no_force_trajectory_omits_force_key(self):
+        """Exercise the complete format path, not just the dump helper."""
+        with tempfile.TemporaryDirectory() as tmpdir:
+            shutil.copy("abacus.md.nostress/INPUT", tmpdir)
+            shutil.copy("abacus.md.nostress/STRU", tmpdir)
+            source_out = "abacus.md.nostress/OUT.autotest"
+            target_out = os.path.join(tmpdir, "OUT.autotest")
+            os.mkdir(target_out)
+            shutil.copy(os.path.join(source_out, "running_md.log"), target_out)
+
+            # Preserve the real trajectory framing while removing only the
+            # optional force columns from the header and atom records.
+            with open(os.path.join(source_out, "MD_dump")) as fp:
+                lines = fp.readlines()
+            with open(os.path.join(target_out, "MD_dump"), "w") as fp:
+                for line in lines:
+                    if "FORCE" in line and "POSITION" in line:
+                        fp.write("INDEX LABEL POSITION (Angstrom)\n")
+                    elif len(line.split()) >= 8 and line.split()[0].isdigit():
+                        fp.write("  " + "  ".join(line.split()[:5]) + "\n")
+                    else:
+                        fp.write(line)
+
+            system = dpdata.LabeledSystem(tmpdir, fmt="abacus/md")
+            self.assertNotIn("forces", system.data)
+
     def test_atom_names(self):
         self.assertEqual(self.system_water.data["atom_names"], ["H", "O"])
         self.assertEqual(self.system_Si.data["atom_names"], ["Si"])