diff --git a/local/pkgs/vasp-gpu/constr_cell_relax.F b/local/pkgs/vasp-gpu/constr_cell_relax.F
new file mode 100644
index 00000000..9939c30a
--- /dev/null
+++ b/local/pkgs/vasp-gpu/constr_cell_relax.F
@@ -0,0 +1,22 @@
+SUBROUTINE CONSTR_CELL_RELAX(FCELL)
+  USE prec
+  REAL(q) FCELL(3,3)
+
+  LOGICAL FILFLG
+  INTEGER ICELL(3,3)
+  INQUIRE(FILE='OPTCELL',EXIST=FILFLG)
+  IF (FILFLG) THEN
+      OPEN(67,FILE='OPTCELL',FORM='FORMATTED',STATUS='OLD')
+      DO J=1,3
+        READ(67,"(3I1)") (ICELL(I,J),I=1,3)
+      ENDDO
+      CLOSE(67)
+      DO J=1,3
+      DO I=1,3
+        IF (ICELL(I,J)==0) FCELL(I,J)=0.0
+      ENDDO
+      ENDDO
+  ENDIF
+
+  RETURN
+END SUBROUTINE
diff --git a/local/pkgs/vasp-gpu/default.nix b/local/pkgs/vasp-gpu/default.nix
index 86e773b3..ea6760ca 100644
--- a/local/pkgs/vasp-gpu/default.nix
+++ b/local/pkgs/vasp-gpu/default.nix
@@ -42,7 +42,11 @@ let
       hashMode = "recursive";
       message = "Source file not found.";
     };
-    configurePhase = "cp ${include version} makefile.include";
+    configurePhase =
+    ''
+      cp ${include version} makefile.include
+      cp ${./constr_cell_relax.F} src/constr_cell_relax.F
+    '';
     enableParallelBuilding = true;
     buildInputs = [ mkl hdf5 wannier90 ];
     nativeBuildInputs = [ gfortran rsync which ];