From 3804849d11e6fbdb7d39500a72d3a8a29e0df936 Mon Sep 17 00:00:00 2001
From: Haoyu Yang <yanghaoyu97@outlook.com>
Date: Sun, 2 Feb 2025 10:01:04 +0100
Subject: [PATCH] add occu_cutoff

---
 src/pymatgen/io/vasp/inputs.py | 27 +++++++++++++++++++++++----
 tests/io/vasp/test_inputs.py   | 11 +++++++++++
 2 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/src/pymatgen/io/vasp/inputs.py b/src/pymatgen/io/vasp/inputs.py
index e4f07494364..a4981661c67 100644
--- a/src/pymatgen/io/vasp/inputs.py
+++ b/src/pymatgen/io/vasp/inputs.py
@@ -2126,11 +2126,17 @@ def __repr__(self) -> str:
         TITEL, VRHFIN, n_valence_elec = (self.keywords.get(key) for key in ("TITEL", "VRHFIN", "ZVAL"))
         return f"{cls_name}({symbol=}, {functional=}, {TITEL=}, {VRHFIN=}, {n_valence_elec=:.0f})"
 
-    @property
-    def electron_configuration(self) -> list[tuple[int, str, float]]:
+    def get_electron_configuration(
+        self,
+        occu_cutoff: float = 0.01,
+    ) -> list[tuple[int, str, float]]:
         """Valence electronic configuration corresponding to the ZVAL,
         read from the "Atomic configuration" section of POTCAR.
 
+        Args:
+            occu_cutoff (float): Occupancy cutoff below which an orbital
+                would be considered empty.
+
         Returns:
             list[tuple[int, str, float]]: A list of tuples containing:
                 - n (int): Principal quantum number.
@@ -2157,11 +2163,11 @@ def electron_configuration(self) -> list[tuple[int, str, float]]:
 
         total_electrons = 0.0
         valence_config: list[tuple[int, str, float]] = []
-        for line in lines[start_idx + 3 + num_entries - 1 : start_idx + 2 : -1]:
+        for line in lines[start_idx + 2 + num_entries : start_idx + 2 : -1]:
             parts = line.split()
             n, ang_moment, _j, _E, occ = int(parts[0]), int(parts[1]), float(parts[2]), float(parts[3]), float(parts[4])
 
-            if occ >= 0.01:  # TODO: hard-coded occupancy cutoff
+            if occ >= occu_cutoff:
                 valence_config.append((n, l_map[ang_moment], occ))
                 total_electrons += occ
 
@@ -2170,6 +2176,19 @@ def electron_configuration(self) -> list[tuple[int, str, float]]:
 
         return list(reversed(valence_config))
 
+    @property
+    def electron_configuration(self) -> list[tuple[int, str, float]]:
+        """Valence electronic configuration corresponding to the ZVAL,
+        read from the "Atomic configuration" section of POTCAR.
+
+        Returns:
+            list[tuple[int, str, float]]: A list of tuples containing:
+                - n (int): Principal quantum number.
+                - subshell (str): Subshell notation (s, p, d, f).
+                - occ (float): Occupation number, limited to ZVAL.
+        """
+        return self.get_electron_configuration()
+
     @property
     def element(self) -> str:
         """Attempt to return the atomic symbol based on the VRHFIN keyword."""
diff --git a/tests/io/vasp/test_inputs.py b/tests/io/vasp/test_inputs.py
index edcfbda33e7..3d2ebb0587c 100644
--- a/tests/io/vasp/test_inputs.py
+++ b/tests/io/vasp/test_inputs.py
@@ -1462,6 +1462,17 @@ def assert_config_equal(actual_config, expected_config) -> None:
             ],
         )
 
+        # Test occupancy cut-off (Be: 2s1.99 2p0.01)
+        assert_config_equal(
+            PotcarSingle.from_file(f"{FAKE_POTCAR_DIR}/POT_GGA_PAW_PBE_54/POTCAR.Be.gz").get_electron_configuration(
+                occu_cutoff=0.1
+            ),
+            [
+                (1, "s", 2.0),
+                (2, "s", 1.99),
+            ],
+        )
+
     def test_attributes(self):
         for key, val in self.Mn_pv_attrs.items():
             assert getattr(self.psingle_Mn_pv, key) == val