diff --git a/src/QuAcK/Ec_AC.f90 b/src/QuAcK/Ec_AC.f90
new file mode 100644
index 0000000..21c94ce
--- /dev/null
+++ b/src/QuAcK/Ec_AC.f90
@@ -0,0 +1,74 @@
+subroutine Ec_AC(ispin,nBas,nC,nO,nV,nR,nS,ERI,XpY,EcAC)
+
+! Compute the correlation energy via the adiabatic connection formula
+
+  implicit none
+  include 'parameters.h'
+
+! Input variables
+
+  integer,intent(in)            :: ispin
+  integer,intent(in)            :: nBas,nC,nO,nV,nR,nS
+  double precision,intent(in)   :: ERI(nBas,nBas,nBas,nBas)
+  double precision,intent(in)   :: XpY(nS,nS)
+
+! Local variables
+
+  integer                       :: i,j,a,b
+  integer                       :: ia,jb,kc
+  double precision              :: delta_spin
+  double precision,allocatable  :: P(:,:)
+
+! Output variables
+
+  double precision,intent(out)  :: EcAC
+
+! Singlet or triplet manifold?
+
+  delta_spin = 0d0
+  if(ispin == 1) delta_spin = +1d0
+  if(ispin == 2) delta_spin = -1d0
+
+! Memory allocation
+
+  allocate(P(nS,nS))
+
+! Compute P = (X+Y)(X+Y) - 1
+
+  P(:,:) = 0d0
+
+  do ia=1,nS
+    do jb=1,nS
+      do kc=1,nS
+
+        P(ia,jb) = P(ia,jb) + XpY(ia,kc)*XpY(kc,jb)
+
+      enddo
+    enddo
+
+    P(ia,ia) = P(ia,ia) - 1d0
+
+  enddo
+
+! Compute Tr[VP]
+
+  EcAC = 0d0
+
+  ia = 0
+  do i=nC+1,nO
+    do a=nO+1,nBas-nR
+      ia = ia + 1
+      jb = 0
+      do j=nC+1,nO
+        do b=nO+1,nBas-nR
+          jb = jb + 1
+
+            EcAC = EcAC + (1d0 + delta_spin)*ERI(i,b,a,j)*P(jb,ia)
+
+        enddo
+      enddo
+    enddo
+  enddo
+
+end subroutine Ec_AC
+