VASP 6.4.2 - quickfix for negative values caused by vacuum region

vasp6.4.2_xclib_grad.F.patch

--- xclib_grad.F
+++ xclib_grad.F
@@ -335,6 +335,15 @@

 #ifdef USELIBXC
       ELSE IF (LEXCH==99) THEN
+! fix negative values caused by vacuum region
+! see: https://www.vasp.at/forum/viewtopic.php?p=25830#p25830
+         IF (D<=0) THEN
+            EXC   = 0._q
+            EXCD  = 0._q
+            EXCDD = 0._q
+            RETURN
+         ENDIF
+
          DTMP(1)=D
          SIGMATMP(1)=DD*DD
          SIGMAZERO(1)=0._q