diff --git a/cmake/Modules/Packages/USER-PACE.cmake b/cmake/Modules/Packages/USER-PACE.cmake
index e8262f7d40..df1fb023a5 100644
--- a/cmake/Modules/Packages/USER-PACE.cmake
+++ b/cmake/Modules/Packages/USER-PACE.cmake
@@ -1,6 +1,6 @@
 
-set(PACELIB_URL "https://github.com/ICAMS/lammps-user-pace/archive/refs/tags/v.2021.2.3.upd2.tar.gz" CACHE STRING "URL for PACE evaluator library sources")
-set(PACELIB_MD5 "8fd1162724d349b930e474927197f20d" CACHE STRING "MD5 checksum of PACE evaluator library tarball")
+set(PACELIB_URL "https://github.com/ICAMS/lammps-user-pace/archive/refs/tags/v.2021.4.9.tar.gz" CACHE STRING "URL for PACE evaluator library sources")
+set(PACELIB_MD5 "4db54962fbd6adcf8c18d46e1798ceb5" CACHE STRING "MD5 checksum of PACE evaluator library tarball")
 mark_as_advanced(PACELIB_URL)
 mark_as_advanced(PACELIB_MD5)
 
diff --git a/lib/pace/Install.py b/lib/pace/Install.py
index 91aa8b3a46..08bbb331bb 100644
--- a/lib/pace/Install.py
+++ b/lib/pace/Install.py
@@ -12,20 +12,22 @@ from argparse import ArgumentParser
 sys.path.append('..')
 from install_helpers import fullpath, geturl, checkmd5sum
 
-parser = ArgumentParser(prog='Install.py',
-                        description="LAMMPS library build wrapper script")
-
 # settings
 
 thisdir = fullpath('.')
-version = "v.2021.2.3.upd2"
+version = 'v.2021.4.9'
 
 # known checksums for different PACE versions. used to validate the download.
 checksums = { \
-        'v.2021.2.3.upd2' : '8fd1162724d349b930e474927197f20d', \
+        'v.2021.2.3.upd2' : '8fd1162724d349b930e474927197f20d', 
+        'v.2021.4.9'      : '4db54962fbd6adcf8c18d46e1798ceb5',          
         }
 
 
+parser = ArgumentParser(prog='Install.py',
+                        description="LAMMPS library build wrapper script")
+
+
 # help message
 
 HELP = """
diff --git a/potentials/Cu-PBE-core-rep.ace b/potentials/Cu-PBE-core-rep.ace
index 338675f718..511bb9af7f 100644
--- a/potentials/Cu-PBE-core-rep.ace
+++ b/potentials/Cu-PBE-core-rep.ace
@@ -1,3 +1,5 @@
+# DATE: 2021-04-08 UNITS: metal CONTRIBUTOR: Yury Lysogorskiy <yury.lysogorskiy@ruhr-uni-bochum.de> CITATION: npj Comp. Mat., submitted (2021)
+
 nelements=1
 elements: Cu
 
diff --git a/src/USER-PACE/pair_pace.cpp b/src/USER-PACE/pair_pace.cpp
index c12753603a..f59291b33e 100644
--- a/src/USER-PACE/pair_pace.cpp
+++ b/src/USER-PACE/pair_pace.cpp
@@ -25,6 +25,7 @@ Copyright 2021 Yury Lysogorskiy^1, Cas van der Oord^2, Anton Bochkarev^1,
 #include "neigh_list.h"
 #include "neigh_request.h"
 #include "neighbor.h"
+#include "update.h"
 
 #include <cmath>
 #include <cstring>
@@ -252,6 +253,10 @@ void PairPACE::settings(int narg, char **arg) {
   if (narg > 1)
     error->all(FLERR,"Illegal pair_style command.");
 
+  // ACE potentials are parameterized in metal units
+  if (strcmp("metal",update->unit_style) != 0)
+    error->all(FLERR,"ACE potentials require 'metal' units");
+
   recursive = true; // default evaluator style: RECURSIVE
   if (narg > 0) {
     if (strcmp(arg[0], RECURSIVE_KEYWORD) == 0)