Add atoms change detection for ASE optimizer compatibility

Track atoms state (positions, cell, numbers, pbc) and detect
changes between calculate() calls. This enables ASE optimizers
like ExpCellFilter to work correctly by triggering new VASP
calculations when the geometry changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jkitchin

vasp/calculator.py

@@ -283,6 +283,49 @@ def set(self, **kwargs) -> dict:
 
         return changed
 
+    def _atoms_changed(self, atoms: Atoms | None) -> bool:
+        """Check if atoms have changed since last calculation.
+
+        Compares positions, cell, and atomic numbers to detect changes
+        that require a new calculation (e.g., for ASE optimizers).
+        """
+        if atoms is None:
+            atoms = self.atoms
+        if atoms is None:
+            return False
+
+        # Check if we have stored state from previous calculation
+        if not hasattr(self, "_last_atoms_state"):
+            return True
+
+        last = self._last_atoms_state
+        try:
+            # Check atomic numbers
+            if not np.array_equal(atoms.numbers, last["numbers"]):
+                return True
+            # Check positions (within tolerance)
+            if not np.allclose(atoms.positions, last["positions"], atol=1e-10):
+                return True
+            # Check cell (within tolerance)
+            if not np.allclose(atoms.cell[:], last["cell"], atol=1e-10):
+                return True
+            # Check pbc
+            if not np.array_equal(atoms.pbc, last["pbc"]):
+                return True
+        except (KeyError, ValueError):
+            return True
+
+        return False
+
+    def _store_atoms_state(self, atoms: Atoms) -> None:
+        """Store current atoms state for change detection."""
+        self._last_atoms_state = {
+            "numbers": atoms.numbers.copy(),
+            "positions": atoms.positions.copy(),
+            "cell": atoms.cell[:].copy(),
+            "pbc": atoms.pbc.copy(),
+        }
+
     def calculate(
         self,
         atoms: Atoms | None = None,
@@ -314,12 +357,17 @@ def calculate(
             self.atoms = atoms
             self._setup_sorting(atoms)
 
+        # Check if atoms have changed (for ASE optimizers)
+        atoms_changed = self._atoms_changed(self.atoms)
+
         # Check current status
         status = self.runner.status(self.directory)
-        log.debug(f"Current status: {status.state}")
+        log.debug(f"Current status: {status.state}, atoms_changed: {atoms_changed}")
 
-        if status.state == JobState.COMPLETE and not self.force:
+        if status.state == JobState.COMPLETE and not self.force and not atoms_changed:
             self.read_results()
+            if self.atoms is not None:
+                self._store_atoms_state(self.atoms)
             return
 
         if status.state == JobState.QUEUED:
@@ -342,6 +390,8 @@ def calculate(
 
         if result.state == JobState.COMPLETE:
             self.read_results()
+            if self.atoms is not None:
+                self._store_atoms_state(self.atoms)
         elif result.state == JobState.FAILED:
             raise VaspNotConverged(result.message or "Calculation failed")