Add a VASP handler (warning) for too high NBANDS (#324)

* Add an auto NBANDS handler

* Fix rounding error

* pre-commit auto-fixes

* pre-commit auto-fixes

* ruff

* fix

* pre-commit auto-fixes

* fx

* Add tests

* pre-commit auto-fixes

* fix test

* fix

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

custodian/vasp/handlers.py

@@ -137,6 +137,7 @@ class VaspErrorHandler(ErrorHandler):
         "coef": ["while reading plane", "while reading WAVECAR"],
         "set_core_wf": ["internal error in SET_CORE_WF"],
         "read_error": ["Error reading item", "Error code was IERR= 5"],
+        "auto_nbands": ["The number of bands has been changed"],
     }
 
     def __init__(
@@ -512,21 +513,7 @@ def correct(self, directory="./"):
 
         if "too_few_bands" in self.errors:
             nbands = None
-            if "NBANDS" in vi["INCAR"]:
-                nbands = vi["INCAR"]["NBANDS"]
-            else:
-                with open(os.path.join(directory, "OUTCAR")) as file:
-                    for line in file:
-                        # Have to take the last NBANDS line since sometimes VASP
-                        # updates it automatically even if the user specifies it.
-                        # The last one is marked by NBANDS= (no space).
-                        if "NBANDS=" in line:
-                            try:
-                                d = line.split("=")
-                                nbands = int(d[-1].strip())
-                                break
-                            except (IndexError, ValueError):
-                                pass
+            nbands = vi["INCAR"]["NBANDS"] if "NBANDS" in vi["INCAR"] else self._get_nbands_from_outcar(directory)
             if nbands:
                 new_nbands = max(int(1.1 * nbands), nbands + 1)  # This handles the case when nbands is too low (< 8).
                 actions.append({"dict": "INCAR", "action": {"_set": {"NBANDS": new_nbands}}})
@@ -737,9 +724,36 @@ def correct(self, directory="./"):
                     )
             self.error_count["algo_tet"] += 1
 
+        if "auto_nbands" in self.errors and (nbands := self._get_nbands_from_outcar(directory)):
+            nelect = load_outcar(os.path.join(directory, "OUTCAR")).nelect
+            if nelect and nbands > 2 * nelect:
+                self.error_count["auto_nbands"] += 1
+                warnings.warn(
+                    "NBANDS seems to be too high. The electronic structure may be inaccurate. "
+                    "You may want to rerun this job with a smaller number of cores.",
+                    UserWarning,
+                )
+
         VaspModder(vi=vi, directory=directory).apply_actions(actions)
         return {"errors": list(self.errors), "actions": actions}
 
+    @staticmethod
+    def _get_nbands_from_outcar(directory: str) -> int | None:
+        nbands = None
+        with open(os.path.join(directory, "OUTCAR")) as file:
+            for line in file:
+                # Have to take the last NBANDS line since sometimes VASP
+                # updates it automatically even if the user specifies it.
+                # The last one is marked by NBANDS= (no space).
+                if "NBANDS=" in line:
+                    try:
+                        d = line.split("=")
+                        nbands = int(d[-1].strip())
+                        break
+                    except (IndexError, ValueError):
+                        pass
+        return nbands
+
 
 class LrfCommutatorHandler(ErrorHandler):
     """