diff --git a/scripts/lib/CIME/XML/env_mach_specific.py b/scripts/lib/CIME/XML/env_mach_specific.py
index ba012966ccb..253082aa34d 100644
--- a/scripts/lib/CIME/XML/env_mach_specific.py
+++ b/scripts/lib/CIME/XML/env_mach_specific.py
@@ -321,7 +321,7 @@ def get_module_system_cmd_path(self, lang):
         cmd_nodes = self.get_optional_node("cmd_path", attributes={"lang":lang})
         return cmd_nodes.text if cmd_nodes is not None else None
 
-    def get_mpirun(self, case, attribs, check_members=None, job="case.run"):
+    def get_mpirun(self, case, attribs, check_members=None, job="case.run", exe_only=False):
         """
         Find best match, return (executable, {arg_name : text})
         """
@@ -372,16 +372,17 @@ def get_mpirun(self, case, attribs, check_members=None, job="case.run"):
         the_match = best_match if best_match is not None else default_match
 
         # Now that we know the best match, compute the arguments
-        arg_node = self.get_optional_node("arguments", root=the_match)
-        if arg_node is not None:
-            arg_nodes = self.get_nodes("arg", root=arg_node)
-            for arg_node in arg_nodes:
-                arg_value = transform_vars(arg_node.text,
-                                           case=case,
-                                           subgroup=job,
-                                           check_members=check_members,
-                                           default=arg_node.get("default"))
-                args[arg_node.get("name")] = arg_value
+        if exe_only:
+            arg_node = self.get_optional_node("arguments", root=the_match)
+            if arg_node is not None:
+                arg_nodes = self.get_nodes("arg", root=arg_node)
+                for arg_node in arg_nodes:
+                    arg_value = transform_vars(arg_node.text,
+                                               case=case,
+                                               subgroup=job,
+                                               check_members=check_members,
+                                               default=arg_node.get("default"))
+                    args[arg_node.get("name")] = arg_value
 
         exec_node = self.get_node("executable", root=the_match)
         expect(exec_node is not None,"No executable found")
diff --git a/scripts/lib/CIME/case.py b/scripts/lib/CIME/case.py
index ecfede3886c..677c80e7987 100644
--- a/scripts/lib/CIME/case.py
+++ b/scripts/lib/CIME/case.py
@@ -123,9 +123,10 @@ def _initialize_derived_attributes(self):
         These are derived variables which can be used in the config_* files
         for variable substitution using the {{ var }} syntax
         """
-        env_mach_pes = self.get_env("mach_pes")
-        comp_classes = self.get_values("COMP_CLASSES")
-        pes_per_node = self.get_value("PES_PER_NODE")
+        env_mach_pes  = self.get_env("mach_pes")
+        env_mach_spec = self.get_env('mach_specific')
+        comp_classes  = self.get_values("COMP_CLASSES")
+        pes_per_node  = self.get_value("PES_PER_NODE")
 
         self.total_tasks = env_mach_pes.get_total_tasks(comp_classes)
         self.thread_count = env_mach_pes.get_max_thread_count(comp_classes)
@@ -139,7 +140,14 @@ def _initialize_derived_attributes(self):
         threads_per_core = 1 if (threads_per_node <= pes_per_node) else smt_factor
         self.cores_per_task = self.thread_count / threads_per_core
 
-        if self.get_value("MACH") == "titan":
+        mpi_attribs = {
+            "compiler" : self.get_value("COMPILER"),
+            "mpilib"   : self.get_value("MPILIB"),
+            "threaded" : get_build_threaded(self)
+            }
+
+        executable = env_mach_spec.get_mpirun(self, mpi_attribs, job="case.run", exe_only=True)[0]
+        if executable == "aprun":
             self.num_nodes = get_aprun_cmd_for_case(self, "acme.exe")[1]
         else:
             self.num_nodes = env_mach_pes.get_total_nodes(self.total_tasks, self.thread_count)
@@ -1068,7 +1076,9 @@ def get_mpirun_cmd(self, job="case.run"):
 
         # special case for aprun
         if executable == "aprun":
-            return get_aprun_cmd_for_case(self, run_exe)[0] + " " + run_misc_suffix
+            aprun_cmd, num_nodes = get_aprun_cmd_for_case(self, run_exe)
+            expect(num_nodes == self.num_nodes, "Not using optimized num nodes")
+            return aprun_cmd + " " + run_misc_suffix
         else:
             mpi_arg_string = " ".join(args.values())