diff --git a/namd.nix b/namd.nix
index a2bd225..67b65d9 100644
--- a/namd.nix
+++ b/namd.nix
@@ -1,26 +1,27 @@
 with import <nixpkgs> {};
 
-stdenv.mkDerivation {
-  name = "namd-2.11";
+stdenv.mkDerivation rec {
+  version = "2.11";
+  name = "namd-${version}";
 
   src = fetchurl {
-    url = http://ppomorsk.sharcnet.ca/namd-2.11.tar.gz;
+    url = "http://ppomorsk.sharcnet.ca/namd-${version}.tar.gz";
     md5 = "8a81eb4c883edbf977d0af05351f6914";
   };
 
   postUnpack = ''
-    cd namd-*
-    tar -xf charm-6.7.0.tar
+    cd namd-${version}
+    tar -xf charm-*.tar
     tar -xzf fftw-linux-x86_64.tar.gz
     mv linux-x86_64 fftw
-    tar -xzf tcl8.5.9-linux-x86_64.tar.gz
-    mv tcl8.5.9-linux-x86_64 tcl
-    tar -xzf tcl8.5.9-linux-x86_64-threaded.tar.gz
-    mv tcl8.5.9-linux-x86_64-threaded tcl-threaded
+    tar -xzf tcl*-linux-x86_64.tar.gz
+    mv tcl*-linux-x86_64 tcl
+    tar -xzf tcl*-linux-x86_64-threaded.tar.gz
+    mv tcl*-linux-x86_64-threaded tcl-threaded
     cd ..
   '';
   patchPhase = ''
-    find charm-6.7.0 -type f | xargs sed -i \
+    find charm-*/ -type f | xargs sed -i \
       -e 's,/bin/cp,cp,g' \
       -e 's,/bin/rm,rm,g' \
       -e 's,/lib/cpp,cpp,g'
@@ -29,7 +30,7 @@ stdenv.mkDerivation {
     tcsh -f ./config  Linux-x86_64-g++   --charm-arch multicore-linux64
   '';
   buildPhase = ''
-    cd charm-6.7.0
+    cd charm-*/
     ./build charm++ multicore-linux64 --no-build-shared --with-production
     cd ..
     cd Linux-x86_64-g++