diff --git a/mctools/generic/get_spacegroup.py b/mctools/generic/get_spacegroup.py
index 95fbfe3..1fbc923 100755
--- a/mctools/generic/get_spacegroup.py
+++ b/mctools/generic/get_spacegroup.py
@@ -2,27 +2,28 @@
 
 import argparse
 import os
+from pathlib import Path
+from typing import Optional
 
 import ase.io
 import spglib
 
 
-def get_spacegroup(filename=False, format=False):
-
-    if filename:
-        pass
-    elif os.path.isfile('geometry.in'):
-        filename = 'geometry.in'
-    elif os.path.isfile('POSCAR'):
-        filename = 'POSCAR'
+def get_default_file() -> Path:
+    for candidate in ("geometry.in", "POSCAR", "castep.cell"):
+        if (input := Path.cwd() / candidate).is_file():
+            return input
     else:
-        raise Exception('No input file!')
+         raise ValueError("Input file not specified, no default found.")
 
-    if format:
-        atoms = ase.io.read(filename, format=format)
-    else:
-        atoms = ase.io.read(filename)
 
+def get_spacegroup(filename: Optional[Path] = None,
+                   format: Optional[str] = None):
+
+    if filename is None:
+        filename = get_default_file()
+
+    atoms = ase.io.read(str(filename), format=format)
     cell = (atoms.cell.array, atoms.get_scaled_positions(), atoms.numbers)
 
     print("| Threshold / Å |    Space group    |")
@@ -35,9 +36,9 @@ def get_spacegroup(filename=False, format=False):
 
 def main():
     parser = argparse.ArgumentParser()
-    parser.add_argument('filename', action='store', default=False,
+    parser.add_argument('filename', type=Path, default=None, nargs="?",
                         help="Input structure file")
-    parser.add_argument('-f', '--format', action='store', default=False,
+    parser.add_argument('-f', '--format', type=str, default=None,
                         help="File format for ASE importer")
     args = parser.parse_args()
     get_spacegroup(filename=args.filename, format=args.format)