diff --git a/src/OohataHuzinaga.jl b/src/OohataHuzinaga.jl
index 7a58044..a5aa96a 100644
--- a/src/OohataHuzinaga.jl
+++ b/src/OohataHuzinaga.jl
@@ -18,6 +18,8 @@ export gaussianproduct
 export normalization
 
 export overlap
+export Operator
+export oei
 export kinetic
 export boys
 export attraction
diff --git a/src/auxiliary.jl b/src/auxiliary.jl
index 798efd0..0b25bcc 100644
--- a/src/auxiliary.jl
+++ b/src/auxiliary.jl
@@ -16,7 +16,6 @@ end
 
 function distance(Rᵢ, Rⱼ)
     d = (Rᵢ[1] - Rⱼ[1])^2 + (Rᵢ[2] - Rⱼ[2])^2 + (Rᵢ[3] - Rⱼ[3])^2
-
     return d
 end
 
diff --git a/src/kinetic.jl b/src/kinetic.jl
index d34a21e..56074e0 100644
--- a/src/kinetic.jl
+++ b/src/kinetic.jl
@@ -1,16 +1,45 @@
+struct Operator<:Function
+    ô
+end
+
 function Kxyz(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ, mᵢ, mⱼ, nᵢ, nⱼ)
     K = αⱼ * (2 * (ℓⱼ + mⱼ + nⱼ) + 3) * Sxyz(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ, mᵢ, mⱼ, nᵢ, nⱼ)
     K -= (2 * (αⱼ^2)) * Sxyz(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ + 2, mᵢ, mⱼ, nᵢ, nⱼ)
     K -= (2 * (αⱼ^2)) * Sxyz(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ, mᵢ, mⱼ + 2, nᵢ, nⱼ)
     K -= (2 * (αⱼ^2)) * Sxyz(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ, mᵢ, mⱼ, nᵢ, nⱼ + 2)
-
     K -= (1 / 2) * (ℓⱼ * (ℓⱼ - 1)) * Sxyz(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ - 2, mᵢ, mⱼ, nᵢ, nⱼ)
     K -= (1 / 2) * (mⱼ * (mⱼ - 1)) * Sxyz(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ, mᵢ, mⱼ - 2, nᵢ, nⱼ)
     K -= (1 / 2) * (nⱼ * (nⱼ - 1)) * Sxyz(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ, mᵢ, mⱼ, nᵢ, nⱼ - 2)
-
     return K
 end
 
+function oei(basis, molecule::Molecule, integral::Operator)
+    n = length(basis)
+    O = zeros(n, n)
+    for c in CartesianIndices(O)
+        i, j = c[1], c[2]
+        basisᵢ, basisⱼ = basis[i], basis[j]
+        Rᵢ, Rⱼ = basisᵢ.R, basisⱼ.R
+        dist = distance(Rᵢ, Rⱼ)
+        m, p = basisᵢ.size, basisⱼ.size
+        ℓᵢ, mᵢ, nᵢ = basisᵢ.ℓ, basisᵢ.m, basisᵢ.n
+        ℓⱼ, mⱼ, nⱼ = basisⱼ.ℓ, basisⱼ.m, basisⱼ.n
+        for e in CartesianIndices((m, p))
+            k, l = e[1], e[2]
+            αᵢ = basisᵢ.α[k]
+            αⱼ = basisⱼ.α[l]
+            dᵢ, dⱼ = basisᵢ.d[k], basisⱼ.d[l]
+            Nᵢ, Nⱼ = basisᵢ.N[k], basisⱼ.N[l]
+            O[i, j] += (
+                exp(-αᵢ * αⱼ * dist / (αᵢ + αⱼ)) *
+                Nᵢ * Nⱼ * dᵢ * dⱼ *
+                integral.ô(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ, mᵢ, mⱼ, nᵢ, nⱼ)
+            )
+        end
+    end
+    return O
+end
+
 function kinetic(basis, molecule::Molecule)
     n = length(basis)
     T = zeros(n, n)
diff --git a/src/overlap.jl b/src/overlap.jl
index 840d1c9..26805de 100644
--- a/src/overlap.jl
+++ b/src/overlap.jl
@@ -1,10 +1,8 @@
 function Sxyz(Rᵢ, Rⱼ, αᵢ, αⱼ, ℓᵢ, ℓⱼ, mᵢ, mⱼ, nᵢ, nⱼ)
     Rₚ = gaussianproduct(αᵢ, Rᵢ, αⱼ, Rⱼ, αᵢ + αⱼ)
-
     Sx = sᵢ(ℓᵢ, ℓⱼ, αᵢ + αⱼ, Rᵢ[1], Rⱼ[1], Rₚ[1])
     Sy = sᵢ(mᵢ, mⱼ, αᵢ + αⱼ, Rᵢ[2], Rⱼ[2], Rₚ[2])
     Sz = sᵢ(nᵢ, nⱼ, αᵢ + αⱼ, Rᵢ[3], Rⱼ[3], Rₚ[3])
-
     return Sx * Sy * Sz
 end