Initial import of htcd system 1.0

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>

1 files changed, 21 insertions, 0 deletions

diff --git a/src/labat/exchen.f b/src/labat/exchen.f
new file mode 100644
index 0000000..a3d1112
--- /dev/null
+++ b/src/labat/exchen.f
@@ -0,0 +1,21 @@
+      subroutine exchen(d,s,ex)
+c  gga91 exchange energy for a spin-unpolarized electronic system
+c  input d   : density
+c  input s   : abs(grad d)/(2*kf*d)
+c  output ex : exchange energy per electron 
+      implicit double precision (a-h,o-z)
+      data a1,a2,a3,a4/0.19645d0,0.27430d0,0.15084d0,100.d0/
+      data ax,a,b1/-0.7385588d0,7.7956d0,0.004d0/
+      data thrd/0.333333333333d0/
+      fac = ax*d**thrd
+      s2 = s*s
+      s4 = s2*s2
+      p0 = 1.d0/dsqrt(1.d0+a*a*s2)
+      p1 = dlog(a*s+1.d0/p0)
+      p2 = dexp(-a4*s2)
+      p3 = 1.d0/(1.d0+a1*s*p1+b1*s4)
+      p4 = 1.d0+a1*s*p1+(a2-a3*p2)*s2
+      f = p3*p4
+      ex = fac*f
+      return
+      end