Update phono.py

Author: bigd4

pynep/phono.py

@@ -116,7 +116,7 @@ def get_supercell_matrix(self, atoms):
         if isinstance(self.dim, str):
             if self.dim == 'Auto':
                 lengths = atoms.cell.lengths()
-                return np.round(10 / lengths).astype('int')
+                return np.round(10 / lengths).astype('int').clip(1)
         else:
             return self.dim