diff --git a/modifiers/allocation/modifier.py b/modifiers/allocation/modifier.py
index d516d653f..bdc2affc0 100644
--- a/modifiers/allocation/modifier.py
+++ b/modifiers/allocation/modifier.py
@@ -57,16 +57,34 @@ def defined_allocation_options(expander):
             val = AllocOpt.as_type(alloc_opt, var_def)
         except ValueError:
             continue
-        defined[alloc_opt] = val
+
+        if val is not None:
+            defined[alloc_opt] = val
 
     return defined
 
 
 class AttrDict(dict):
-    __getattr__ = dict.__getitem__
-    __setattr__ = dict.__setitem__
-    __delattr__ = dict.__delitem__
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self['_attributes'] = set()
+
+    def __getattr__(self, *args, **kwargs):
+        return self.__getitem__(*args, **kwargs)
+
+    def __setattr__(self, *args, **kwargs):
+        self.__setitem__(*args, **kwargs)
+
+    def __delattr__(self, *args, **kwargs):
+        self.__delitem__(*args, **kwargs)
+
+    def __setitem__(self, key, value):
+        super().__setitem__(key, value)
+        if key != "_attributes":
+            self['_attributes'].add(key)
 
+    def defined(self):
+        return list((k, self[k]) for k in self['_attributes'])
 
 class Allocation(BasicModifier):
 
@@ -86,14 +104,15 @@ def inherit_from_application(self, app):
 
         var_defs = defined_allocation_options(app.expander)
 
-        # Calculate unset values (e.g. determine n_nodes if not set)
-        self.determine_allocation(var_defs)
+        with Allocation.as_attrs(var_defs) as v:
+            # Calculate unset values (e.g. determine n_nodes if not set)
+            self.determine_allocation(v)
 
-        self.determine_scheduler_instructions(var_defs)
+            self.determine_scheduler_instructions(v)
 
-        # Definitions
-        for var, val in var_defs.items():
-            app.define_variable(var.name.lower(), str(val))
+            # Definitions
+            for var, val in v.defined():
+                app.define_variable(var, str(val))
 
     @staticmethod
     @contextmanager
@@ -104,29 +123,23 @@ def as_attrs(var_defs):
 
         yield v
 
-        for alloc_opt in AllocOpt:
-            local_val = getattr(v, alloc_opt.name.lower(), None)
-            if (alloc_opt not in var_defs) and local_val:
-                var_defs[alloc_opt] = local_val
+    def determine_allocation(self, v):
+        if not v.n_ranks:
+            if v.n_ranks_per_node and v.n_nodes:
+                v.n_ranks = v.n_nodes * v.n_ranks_per_node
 
-    def determine_allocation(self, var_defs):
-        with Allocation.as_attrs(var_defs) as v:
-            if not v.n_ranks:
-                if v.n_ranks_per_node and v.n_nodes:
-                    v.n_ranks = v.n_nodes * v.n_ranks_per_node
-
-            if not v.n_nodes:
-                if v.n_ranks:
-                    cpus_request_per_task = 1
-                    multi_cpus_per_task = n_cores_per_task or n_threads or 0
-                    cpus_request_per_task = max(multi_cpus_per_task, 1)
-                    tasks_per_node = math.floor(cpus_per_node / cpus_request_per_task)
-                    v.n_nodes = math.ceil(v.n_ranks / tasks_per_node)
-                if v.n_gpus and v.gpus_per_node:
-                    v.n_nodes = math.ceil(v.n_gpus / float(v.gpus_per_node))
-
-            if not v.n_threads:
-                v.n_threads = 1
+        if not v.n_nodes:
+            if v.n_ranks:
+                cpus_request_per_task = 1
+                multi_cpus_per_task = v.n_cores_per_task or v.n_threads or 0
+                cpus_request_per_task = max(multi_cpus_per_task, 1)
+                tasks_per_node = math.floor(cpus_per_node / cpus_request_per_task)
+                v.n_nodes = math.ceil(v.n_ranks / tasks_per_node)
+            if v.n_gpus and v.gpus_per_node:
+                v.n_nodes = math.ceil(v.n_gpus / float(v.gpus_per_node))
+
+        if not v.n_threads:
+            v.n_threads = 1
 
     def slurm_instructions(self, v):
         srun_opts = []
@@ -134,9 +147,9 @@ def slurm_instructions(self, v):
         if v.n_ranks:
             srun_opts.append(f"-n {v.n_ranks}")
         if v.n_gpus:
-            srun_opts.append(f"-n {v.n_gpus}")
+            srun_opts.append(f"--gpus {v.n_gpus}")
         if v.n_nodes:
-            srun_opts.append(f"-n {v.n_nodes}")
+            srun_opts.append(f"-N {v.n_nodes}")
 
         sbatch_directives = list(f"# SBATCH {x}" for x in srun_opts)
 
@@ -152,11 +165,12 @@ def mpi_instructions(self, v):
         v.batch_submit = "{execute_experiment}"
         v.allocation_directives = ""
 
-    def determine_scheduler_instructions(self, var_defs):
+    def determine_scheduler_instructions(self, v):
         handler = {
             "slurm": self.slurm_instructions,
             "flux": self.flux_instructions,
             "mpi": self.mpi_instructions
         }
-        with Allocation.as_attrs(var_defs) as v:
-            handler[v.scheduler](v)
+        if v.scheduler not in handler:
+            raise ValueError(f"scheduler ({v.scheduler}) must be one of : " + " ".join(handler.keys()))
+        handler[v.scheduler](v)