Add error for old castep_bin files

CHANGELOG.rst

@@ -1,10 +1,21 @@
 `Unreleased <https://github.com/pace-neutrons/Euphonic/compare/v1.3.1...HEAD>`_
 
+- Requirements
+
+  - ``packaging`` library added to dependencies.
+
 - Bug fixes
 
   - Fixed an error loading QpointPhononModes from JSON when there is a
     single q-point in the data
 
+- Improvements
+
+  - When loading ``.castep_bin`` files, explicitly check the CASTEP
+    version number and give a useful error message if this is < 17.1.
+    (These files are missing information about the unit cell origins,
+    and would previously cause an error with an unhelpful message.)
+
 - Maintenance
 
   - Compatibility fix for spglib 2.4 update: a new sanity-check in

euphonic/readers/castep.py

@@ -1,4 +1,5 @@
 from collections import defaultdict
+from packaging.version import Version
 import re
 import struct
 from typing import (Any, BinaryIO, Dict, Iterator, List, NamedTuple,
@@ -451,6 +452,17 @@ def read_interpolation_data(
         float_type = '>f8'
         first_cell_read = True
         while (header := _read_entry(f).strip()) != b'END':
+            if header == b'BEGIN_PARAMETERS_DUMP':
+                castep_version: Version = Version(_read_entry(f).decode().strip())
+                if castep_version < Version("17.1"):
+                    raise ValueError('Old castep file detected: '
+                        'Euphonic only supports post-Castep 17.1 files. '
+                        'Please rerun the calculation with a newer version '
+                        'of Castep with the original .cell file and a '
+                        '.castep file with a single line with the '
+                        '"continuation: <old.castep_bin>" keyword and '
+                        'use the new output .castep_bin file in Euphonic.')
+
             if header == b'BEGIN_UNIT_CELL':
                 # CASTEP writes the cell twice: the first is the
                 # geometry optimised cell, the second is the original
@@ -469,8 +481,10 @@ def read_interpolation_data(
                     np.reshape(_read_entry(f, float_type),
                                (n_cells_in_sc, 3*n_atoms, 3*n_atoms)),
                     axes=[0, 2, 1]))
-                cell_origins = np.reshape(
-                    _read_entry(f, int_type), (n_cells_in_sc, 3))
+
+                cell_origins = np.reshape(_read_entry(f, int_type),
+                                          (n_cells_in_sc, 3))
+
                 _ = _read_entry(f, int_type)  # FC row not used
             elif header == b'BORN_CHGS':
                 born = np.reshape(

setup.py

@@ -138,6 +138,7 @@ def run_setup():
         packages=packages,
         include_package_data=True,
         install_requires=[
+            'packaging',
             'scipy>=1.10',  # requires numpy >= 1.19.5
             'seekpath>=1.1.0',
             'spglib>=1.9.4',