diff --git a/config-hlrs/packages.yaml b/config-hlrs/packages.yaml
index 71db90e..a9b6de8 100644
--- a/config-hlrs/packages.yaml
+++ b/config-hlrs/packages.yaml
@@ -6,6 +6,7 @@ packages:
       lapack: [cray-libsci]
       scalapack: [cray-libsci]
       pkgconf: [pkg-config]
+      zlib-ng: [zlib]
   cray-mpich:
     externals:
     - spec: cray-mpich@8.1.30%gcc
@@ -33,6 +34,32 @@ packages:
       modules:
       - cray-libsci/24.03.0
     buildable: false
+# Packages which have not been discovered by spack external find 
+# and packages which are not in the OS image but low-level.
+# Declare libs as externals only if a -devel OS package is installed
+  elfutils:
+    externals:
+    - spec: elfutils@0.185
+      prefix: /usr
+    buildable: false
+  numactl:
+    externals:
+    - spec: numactl@2.0.14
+      prefix: /usr
+    buildable: false
+  libxml2:
+    externals:
+    - spec: libxml2@2.9.14
+    buildable: false
+  libdwarf:
+    require: '%gcc@7.5'
+  zstd:
+    require: '%gcc@7.5'
+  zlib:     # zlib@1.2.11 is available but deprecated
+    require: '%gcc@7.5'
+  zlib-ng:
+    require: '%gcc@7.5'
+# packages which have been discovered by spack external find --all
   python:
     externals:
     - spec: python@3.6.15+bz2+crypt+ctypes+dbm+lzma+nis+pyexpat~pythoncmd+readline+sqlite3+ssl~tkinter+uuid+zlib
diff --git a/environments/hlrs/spack.yaml b/environments/hlrs/spack.yaml
index 38eacec..686f6aa 100644
--- a/environments/hlrs/spack.yaml
+++ b/environments/hlrs/spack.yaml
@@ -3,13 +3,9 @@ spack:
 #  definitions:
 #    core_compiler: [gcc@7.5]
 #    compilers: [gcc@13.2, cce@18.0]    
-  packages:
+#  packages:
 #    all:
 #      compiler: [gcc@13.2, cce@18.0]
- # force OS related libs to use core compiler
-    libdwarf:
-      require:
-      - '%gcc@7.5'
   # add package specs to the `specs` list
   specs:
     - matrix:    # modules for low-level tools