diff --git a/src/core/GREX.cpp b/src/core/GREX.cpp
index 871fc9af0ef7108619da2e75caafb2bfbed12d9b..001b0108f600389c80a990416d756b18123b77ec 100644
--- a/src/core/GREX.cpp
+++ b/src/core/GREX.cpp
@@ -25,14 +25,15 @@
 #include "tools/Tools.h"
 #include "tools/Communicator.h"
 #include <sstream>
+#include <unordered_map>
 
 #include "GREXEnum.inc"
 
 using namespace std;
 namespace PLMD{
 
-std::map<std::string, int> & GREXWordMap(){
-  static std::map<std::string, int> word_map;
+const std::unordered_map<std::string, int> & GREXWordMap(){
+  static std::unordered_map<std::string, int> word_map;
   static bool init=false;
   if(!init){
 #include "GREXMap.inc"
@@ -73,7 +74,7 @@ void GREX::cmd(const string&key,void*val){
     // do nothing
   } else {
     int iword=-1;
-    std::map<std::string, int>::const_iterator it=GREXWordMap().find(words[0]);
+    auto it=GREXWordMap().find(words[0]);
     if(it!=GREXWordMap().end()) iword=it->second;
     switch(iword){
     case cmd_initialized:
diff --git a/src/core/PlumedMain.cpp b/src/core/PlumedMain.cpp
index 007cdf8ae8a6c534fdb3792a19bf5bcd7edf8806..e391a999a4fca189677d17afa1de3712a7f10940 100644
--- a/src/core/PlumedMain.cpp
+++ b/src/core/PlumedMain.cpp
@@ -43,6 +43,7 @@
 #include <cstdlib>
 #include <cstring>
 #include <set>
+#include <unordered_map>
 
 using namespace std;
 
@@ -50,8 +51,8 @@ using namespace std;
 
 namespace PLMD{
 
-std::map<std::string, int> & plumedMainWordMap(){
-  static std::map<std::string, int> word_map;
+const std::unordered_map<std::string, int> & plumedMainWordMap(){
+  static std::unordered_map<std::string, int> word_map;
   static bool init=false;
   if(!init){
 #include "PlumedMainMap.inc"
@@ -127,7 +128,7 @@ void PlumedMain::cmd(const std::string & word,void*val){
   } else {
     int iword=-1;
     double d;
-    std::map<std::string, int>::const_iterator it=plumedMainWordMap().find(words[0]);
+    auto it=plumedMainWordMap().find(words[0]);
     if(it!=plumedMainWordMap().end()) iword=it->second;
     switch(iword) {
       case cmd_setBox: