diff --git a/builder.sh b/builder.sh
deleted file mode 100755
index 987d585..0000000
--- a/builder.sh
+++ /dev/null
@@ -1,41 +0,0 @@
-source $stdenv/setup
-
-tar xvfz $src
-cd namd-*
-
-tar xvf charm-6.7.0.tar
-cd charm-6.7.0
-
-find . -type f |xargs sed -i 's/\/bin\/cp/cp/g'
-find . -type f |xargs sed -i 's/\/bin\/rm/rm/g'
-find . -type f |xargs sed -i 's/\/lib\/cpp/cpp/g'
-
-./build charm++ multicore-linux64 --no-build-shared --with-production
-cd ..
-
-tar xvfz fftw-linux-x86_64.tar.gz
-mv linux-x86_64 fftw
-tar xvfz tcl8.5.9-linux-x86_64.tar.gz
-mv tcl8.5.9-linux-x86_64 tcl
-tar xvfz tcl8.5.9-linux-x86_64-threaded.tar.gz
-mv tcl8.5.9-linux-x86_64-threaded tcl-threaded
-
-tcsh -f ./config  Linux-x86_64-g++   --charm-arch multicore-linux64
-cd Linux-x86_64-g++
-make
-rm -rf obj
-rm -rf src
-rm -rf inc
-rm Make*
-rm -rf plugins
-cp -r ../plugins .
-rm sb
-cp -r ../psfgen ./sb
-mkdir -p $out/lib/namd
-cp -r * $out/lib/namd
-cd ..
-
-mkdir -p $out/bin
-ln -s $out/lib/namd2 $out/bin/namd2
-
-
diff --git a/namd.nix b/namd.nix
index f0041b2..a2bd225 100644
--- a/namd.nix
+++ b/namd.nix
@@ -1,13 +1,51 @@
 with import <nixpkgs> {};
 
-stdenv.mkDerivation { 
-  name = "namd-2.11"; 
-  builder = ./builder.sh; 
-  src = fetchurl { 
+stdenv.mkDerivation {
+  name = "namd-2.11";
+
+  src = fetchurl {
     url = http://ppomorsk.sharcnet.ca/namd-2.11.tar.gz;
     md5 = "8a81eb4c883edbf977d0af05351f6914";
   };
 
+  postUnpack = ''
+    cd namd-*
+    tar -xf charm-6.7.0.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
+    cd ..
+  '';
+  patchPhase = ''
+    find charm-6.7.0 -type f | xargs sed -i \
+      -e 's,/bin/cp,cp,g' \
+      -e 's,/bin/rm,rm,g' \
+      -e 's,/lib/cpp,cpp,g'
+  '';
+  configurePhase = ''
+    tcsh -f ./config  Linux-x86_64-g++   --charm-arch multicore-linux64
+  '';
+  buildPhase = ''
+    cd charm-6.7.0
+    ./build charm++ multicore-linux64 --no-build-shared --with-production
+    cd ..
+    cd Linux-x86_64-g++
+    make
+    cd ..
+  '';
+  installPhase = ''
+    rm -fr Linux-x86_64-g++/{obj,src,inc,Make*,plugins,sb}
+    cp -r plugins Linux-x86_64-g++/
+    cp -r psfgen Linux-x86_64-g++/sb
+    mkdir -p $out/lib/namd
+    cp -r Linux-x86_64-g++/* $out/lib/namd
+    mkdir -p $out/bin
+    ln -s $out/lib/namd/namd2 $out/bin/namd2
+  '';
+
   buildInputs = [
     perl
     bash