Update inequality in get_linear_interpolated_value (#4299)

* Update inequality in `get_linear_interpolated_value`

* Add test for updated edge case behaviour

* Rename `firs` to `fict_ionic_radii` for clarity and to avoid `codespell` errors in linting

* Add `if not below_fermi or not above_fermi:` catch to `DOS.get_interpolated_gap()` (to be consistent with `Dos.get_interpolated_gap()`)

* Return zero gap for case of VBM index = CBM index -1 (i.e. no gap found)

Author: kavanase

src/pymatgen/analysis/local_env.py

@@ -3745,23 +3745,23 @@ def get_nn_info(self, structure: Structure, n: int):
 
         if self.use_fictive_radius:
             # calculate fictive ionic radii
-            firs = [_get_fictive_ionic_radius(site, neighbor) for neighbor in neighbors]
+            fict_ionic_radii = [_get_fictive_ionic_radius(site, neighbor) for neighbor in neighbors]
         else:
             # just use the bond distance
-            firs = [neighbor.nn_distance for neighbor in neighbors]
+            fict_ionic_radii = [neighbor.nn_distance for neighbor in neighbors]
 
         # calculate mean fictive ionic radius
-        mefir = _get_mean_fictive_ionic_radius(firs)
+        mefir = _get_mean_fictive_ionic_radius(fict_ionic_radii)
 
         # iteratively solve MEFIR; follows equation 4 in Hoppe's EconN paper
         prev_mefir = float("inf")
         while abs(prev_mefir - mefir) > 1e-4:
             # this is guaranteed to converge
             prev_mefir = mefir
-            mefir = _get_mean_fictive_ionic_radius(firs, minimum_fir=mefir)
+            mefir = _get_mean_fictive_ionic_radius(fict_ionic_radii, minimum_fir=mefir)
 
         siw = []
-        for nn, fir in zip(neighbors, firs, strict=True):
+        for nn, fir in zip(neighbors, fict_ionic_radii, strict=True):
             if nn.nn_distance < self.cutoff:
                 w = math.exp(1 - (fir / mefir) ** 6)
                 if w > self.tol:

src/pymatgen/electronic_structure/dos.py

@@ -107,9 +107,12 @@ def get_interpolated_gap(
         energies = self.x
         below_fermi = [i for i in range(len(energies)) if energies[i] < self.efermi and tdos[i] > tol]
         above_fermi = [i for i in range(len(energies)) if energies[i] > self.efermi and tdos[i] > tol]
+        if not below_fermi or not above_fermi:
+            return 0.0, self.efermi, self.efermi
+
         vbm_start = max(below_fermi)
         cbm_start = min(above_fermi)
-        if vbm_start == cbm_start:
+        if vbm_start in [cbm_start, cbm_start - 1]:
             return 0.0, self.efermi, self.efermi
 
         # Interpolate between adjacent values
@@ -311,7 +314,7 @@ def get_interpolated_gap(
 
         vbm_start = max(below_fermi)
         cbm_start = min(above_fermi)
-        if vbm_start == cbm_start:
+        if vbm_start in [cbm_start, cbm_start - 1]:
             return 0.0, self.efermi, self.efermi
 
         # Interpolate between adjacent values

src/pymatgen/util/coord.py

@@ -135,7 +135,7 @@ def get_linear_interpolated_value(x_values: ArrayLike, y_values: ArrayLike, x: f
     """
     arr = np.array(sorted(zip(x_values, y_values, strict=True), key=lambda d: d[0]))
 
-    indices = np.where(arr[:, 0] >= x)[0]
+    indices = np.where(arr[:, 0] > x)[0]
 
     if len(indices) == 0 or indices[0] == 0:
         raise ValueError(f"{x=} is out of range of provided x_values ({min(x_values)}, {max(x_values)})")

tests/util/test_coord.py

@@ -19,6 +19,11 @@ def test_get_linear_interpolated_value(self):
         with pytest.raises(ValueError, match=r"x=6 is out of range of provided x_values \(0, 5\)"):
             coord.get_linear_interpolated_value(x_vals, y_vals, 6)
 
+        # test when x is equal to first value in x_vals (previously broke, fixed in #4299):
+        assert coord.get_linear_interpolated_value(x_vals, y_vals, 0) == approx(3)
+        with pytest.raises(ValueError, match=r"x=-0.5 is out of range of provided x_values \(0, 5\)"):
+            coord.get_linear_interpolated_value(x_vals, y_vals, -0.5)
+
     def test_in_coord_list(self):
         coords = [[0, 0, 0], [0.5, 0.5, 0.5]]
         test_coord = [0.1, 0.1, 0.1]