do not allow melting when lattice is fixed

calphy/liquid.py

@@ -56,7 +56,11 @@ def __init__(self, calculation=None, simfolder=None, log_to_screen=False):
 
     def melt_structure(self, lmp):
         """
-        """        
+        """
+        if self.calc._fix_lattice and self.calc.melting_cycle:
+            raise ValueError("Cannot fix lattice and melt structure (set to False) at the same time")
+
+
         melted = False
 
         #this is the multiplier for thigh to try melting routines