Get FC import from CASTEP 25.1 working (#364)

- CASTEP 25.1 adds extra fields to BORN and DIELECTRIC sections of
  castep_bin file; read (and discard) this information

- It can also create a BORN section without corresponding
  DIELECTRIC (by Berry Phase calculation). That throws off some Euphonic
  logic which assumes you would only ever have BORN and DIELECTRIC data
  at the same time.

- As FC behaviour isn't really defined when you only have one, let's
  discard it in that case.

CHANGELOG.rst

@@ -1,11 +1,29 @@
 `Unreleased <https://github.com/pace-neutrons/Euphonic/compare/v1.4.0.post1...HEAD>`_
 -------------------------------------------------------------------------------------
 
-- The euphonic.spectra module has been broken up into a subpackage
-  with the single-spectrum classes defined in euphonic.spectra.base
-  and the collections in euphonic.spectra.collections. This is not a
-  breaking change: the public classes, functions and type annotations
-  remain importable from euphonic.spectra.
+- Bug fixes
+
+  - CASTEP 25.1 allows Born effective charges to be calculated by
+    Berry Phase methods without a corresponding dielectric tensor. In
+    such cases, no long-range term can be subtracted from the Force
+    Constants (or reconstructed).  Euphonic uses the presence of Born
+    effective charges to indicate such a subtraction; to prevent
+    ill-defined cases, ForceConstants now sets both Born charges and
+    dielectric tensor to None if only one was provided.
+
+- Maintenance
+
+  - The euphonic.spectra module has been broken up into a subpackage
+    with the single-spectrum classes defined in euphonic.spectra.base
+    and the collections in euphonic.spectra.collections. This is not a
+    breaking change: the public classes, functions and type annotations
+    remain importable from euphonic.spectra.
+
+  - CASTEP 25.1 includes an extra field in .castep_bin files,
+    indicating whether Born effective charges were read from an
+    external file. For clarity and safety, this field is now
+    explicitly read by the Euphonic .castep_bin parser, but remains unused.
+
 
 - Bug fixes
 

euphonic/force_constants.py

@@ -108,16 +108,16 @@ def __init__(self, crystal: Crystal, force_constants: Quantity,
         self.cell_origins = cell_origins
         self.n_cells_in_sc = n_sc
 
-        if born is not None:
+        if born is not None and dielectric is not None:
             self._born = born.to(ureg.e).magnitude
             self.born_unit = str(born.units)
             self._dielectric = dielectric.to(ureg(
                 'e**2/(bohr*hartree)')).magnitude
             self.dielectric_unit = str(dielectric.units)
         else:
-            self._born = born
+            self._born = None
             self.born_unit = str(ureg.e)
-            self._dielectric = dielectric
+            self._dielectric = None
             self.dielectric_unit = str(ureg((
                 'e**2/(bohr*hartree)')))
 
@@ -473,7 +473,7 @@ def _calculate_phonons_at_qpts(
             [weights], [[np.ndarray, type(None)]], [(len(qpts),)], ['weights'])
 
         # Set default splitting params
-        if self.born is None:
+        if self.born is None or self.dielectric is None:
             dipole = False
         if not dipole:
             splitting = False

euphonic/readers/castep.py

@@ -491,9 +491,19 @@ def read_interpolation_data(
             elif header == b'BORN_CHGS':
                 born = np.reshape(
                     _read_entry(f, float_type), (n_atoms, 3, 3))
+
+                if castep_version > Version("25.1"):
+                    # Extra field marks if born charges were read from BORN
+                    _ = _read_entry(f)
+
             elif header == b'DIELECTRIC':
+                entry = _read_entry(f, float_type)
                 dielectric = np.transpose(np.reshape(
-                    _read_entry(f, float_type), (3, 3)))
+                    entry, (3, 3)))
+
+                if castep_version > Version("25.1"):
+                    # Extra field marks if dielectric tensor was read from BORN
+                    _ = _read_entry(f)
 
     data_dict: Dict[str, Any] = {}
     cry_dict = data_dict['crystal'] = {}
@@ -528,6 +538,7 @@ def read_interpolation_data(
         data_dict['dielectric'] = dielectric*ureg(
             'e**2/(hartree*bohr)').to(dielectric_unit).magnitude
         data_dict['dielectric_unit'] = dielectric_unit
+
     except UnboundLocalError:
         pass