Default to None for charge_difference and check there are enough waters.

src/somd2/config/_config.py

@@ -104,7 +104,7 @@ def __init__(
         perturbable_constraint="h_bonds_not_heavy_perturbed",
         include_constrained_energies=False,
         dynamic_constraints=True,
-        charge_difference=0,
+        charge_difference=None,
         com_reset_frequency=10,
         minimise=True,
         equilibration_time="0ps",
@@ -877,7 +877,10 @@ def charge_difference(self):
     def charge_difference(self, charge_difference):
         if charge_difference is not None:
             if not isinstance(charge_difference, int):
-                raise ValueError("'charge_difference' must be an integer")
+                try:
+                    charge_difference = int(charge_difference)
+                except:
+                    raise ValueError("'charge_difference' must be an integer")
         self._charge_difference = charge_difference
 
     @property

src/somd2/runner/_runner.py

@@ -183,14 +183,22 @@ def __init__(self, system, config):
 
         # Make sure the charge difference matches the expected value
         # from the config.
-        if self._config.charge_difference != charge_diff:
+        if (
+            self._config.charge_difference is not None
+            and self._config.charge_difference != charge_diff
+        ):
             _logger.warning(
                 f"The charge difference of {charge_diff} between the end states "
                 f"does not match the specified value of {self._config.charge_difference}"
             )
+        else:
+            _logger.info(
+                f"There is a charge difference of {charge_diff} between the end states. "
+                f"Adding alchemical ions to keep the charge constant."
+            )
 
         # The user value takes precedence.
-        if self._config.charge_difference != 0:
+        if self._config.charge_difference is not None:
             charge_diff = self._config.charge_difference
 
         # Create alchemical ions.
@@ -395,6 +403,14 @@ def _create_alchemical_ions(system, charge_diff):
         # The number of waters to convert is the absolute charge difference.
         num_waters = abs(charge_diff)
 
+        # Make sure there are enough waters to convert. The charge difference should
+        # never be this large, but it prevents a crash if it is.
+        if num_waters > len(system["water"].molecules()):
+            raise ValueError(
+                f"Insufficient waters to convert to ions. {num_waters} required, "
+                f"{len(system['water'].molecules())} available."
+            )
+
         # Reference coordinates.
         coords = system.molecules("property is_perturbable").coordinates()
         coord_string = f"{coords[0].value()}, {coords[1].value()}, {coords[2].value()}"