diff --git a/src/UNIFAC.jl b/src/UNIFAC.jl
index c2cce4b..0635892 100644
--- a/src/UNIFAC.jl
+++ b/src/UNIFAC.jl
@@ -1,5 +1,200 @@
+# Thomas A. Christensen II
+# Spring 2020
+# UNIFAC Solver
+
 module UNIFAC
 
-greet() = print("Hello World!")
+export unifac
+
+function unifac(ν, x, T)
+# unifac takes three arguments:
+# ν (that's the Greek lowercase nu) is a 42 x n Integer array where n is the
+#   number of species in the system. The number at ν[k,i] indicates how many
+#   instances of subgroup k are present in a single molecule of species i
+# x is a 1 x n Float64 array that contains the liquid species mole fractions
+# T is the temperature of the system in Kelvins
+
+    # Declare the UNIFAC subgroup parameters
+    R = Array{Float64}(undef, 42, 1)
+    R[1]  = 0.9011
+    R[2]  = 0.6744
+    R[3]  = 0.4469
+    R[4]  = 0.2195
+    R[10] = 0.5313
+    R[12] = 1.2663
+    R[13] = 1.0396
+    R[15] = 1.0000
+    R[17] = 0.9200
+    R[19] = 1.6724
+    R[20] = 1.4457
+    R[25] = 1.1450
+    R[26] = 0.9183
+    R[27] = 0.6908
+    R[32] = 1.4337
+    R[33] = 1.2070
+    R[34] = 0.9795
+    R[41] = 1.8701
+    R[42] = 1.6434
+
+    Q = Array{Float64}(undef, 42, 1)
+    Q[1]  = 0.848
+    Q[2]  = 0.540
+    Q[3]  = 0.228
+    Q[4]  = 0.000
+    Q[10] = 0.400
+    Q[12] = 0.968
+    Q[13] = 0.660
+    Q[15] = 1.200
+    Q[17] = 1.400
+    Q[19] = 1.488
+    Q[20] = 1.180
+    Q[25] = 1.088
+    Q[26] = 0.780
+    Q[27] = 0.468
+    Q[32] = 1.244
+    Q[33] = 0.936
+    Q[34] = 0.624
+    Q[41] = 1.724
+    Q[42] = 1.416
+
+    # Declare the subgroup interaction parameters
+    a = zeros(42, 42)
+    a[1:4, 10]    .= 61.13
+    a[1:4, 12:13] .= 76.50
+    a[1:4, 15]    .= 986.50
+    a[1:4, 17]    .= 1318.00
+    a[1:4, 19:20] .= 476.40
+    a[1:4, 25:27] .= 251.50
+    a[1:4, 32:34] .= 255.70
+    a[1:4, 41:42] .= 597.00
+
+    a[10, 1:4]   .= -11.12
+    a[10, 12:13] .= 167.00
+    a[10, 15]     = 636.10
+    a[10, 17]     = 903.80
+    a[10, 19:20] .= 25.77
+    a[10, 25:27] .= 32.14
+    a[10, 32:34] .= 122.80
+    a[10, 41:42] .= 212.50
+
+    a[12:13, 1:4]   .= -69.70
+    a[12:13, 10]    .= -146.80
+    a[12:13, 15]    .= 803.20
+    a[12:13, 17]    .= 5695.00
+    a[12:13, 19:20] .= -52.10
+    a[12:13, 25:27] .= 213.10
+    a[12:13, 32:34] .= -49.29
+    a[12:13, 41:42] .= 6096.00
+
+    a[15, 1:4]   .= 156.40
+    a[15, 10]     = 89.60
+    a[15, 12:13] .= 25.82
+    a[15, 17]     = 353.50
+    a[15, 19:20] .= 84.00
+    a[15, 25:27] .= 28.06
+    a[15, 32:34] .= 42.70
+    a[15, 41:42] .= 6.712
+
+    a[17, 1:4]   .= 300.00
+    a[17, 10]     = 362.30
+    a[17, 12:13] .= 377.60
+    a[17, 15]     = -229.10
+    a[17, 19:20] .= -195.40
+    a[17, 25:27] .= 540.50
+    a[17, 32:34] .= 168.00
+    a[17, 41:42] .= 112.60
+
+    a[19:20, 1:4]   .= 26.79
+    a[19:20, 10]    .= 140.10
+    a[19:20, 12:13] .= 365.80
+    a[19:20, 15]    .= 164.50
+    a[19:20, 17]    .= 472.50
+    a[19:20, 25:27] .= -103.60
+    a[19:20, 32:34] .= -174.20
+    a[19:20, 41:42] .= 481.70
+
+    a[25:27, 1:4]   .= 83.36
+    a[25:27, 10]    .= 52.13
+    a[25:27, 12:13] .= 65.69
+    a[25:27, 15]    .= 237.70
+    a[25:27, 17]    .= -314.70
+    a[25:27, 19:20] .= 191.10
+    a[25:27, 32:34] .= 251.50
+    a[25:27, 41:42] .= -18.51
+
+    a[32:34, 1:4]   .= 65.33
+    a[32:34, 10]    .= -22.31
+    a[32:34, 12:13] .= 223.00
+    a[32:34, 15]    .= -150.00
+    a[32:34, 17]    .= -448.20
+    a[32:34, 19:20] .= 394.60
+    a[32:34, 25:27] .= -56.08
+    a[32:34, 41:42] .= 147.10
+
+    a[41:42, 1:4]   .= 24.82
+    a[41:42, 10]    .= -22.97
+    a[41:42, 12:13] .= -138.40
+    a[41:42, 15]    .= 185.40
+    a[41:42, 17]    .= 242.80
+    a[41:42, 19:20] .= -287.50
+    a[41:42, 25:27] .= 38.81
+    a[41:42, 32:34] .= -108.50
+
+    # Calculate r
+    r = zeros(1, size(ν, 2))
+    for i in 1:size(ν, 2)
+        r[i] = sum(ν[:, i] .* R)
+    end
+
+    # Calculate q
+    q = zeros(1, size(ν, 2))
+    for i in 1:size(ν, 2)
+        q[i] = sum(ν[:, i] .* Q)
+    end
+
+    # Calculate e
+    e = zeros(42, size(ν, 2))
+    for i in 1:size(ν,2 )
+        e[:, i] = (ν[:, i] .* Q) ./ q[i]
+    end
+
+    # Calculate tau
+    τ = exp.(-a ./ T)
+
+    # Calculate beta
+    β = zeros(42, size(ν, 2))
+    for i in 1:size(ν, 2)
+        for k in 1:42
+            β[k, i] = sum(e[:, i] .* τ[:, k])
+        end
+    end
+
+    # Calculate Theta
+    Θ = zeros(42, 1)
+    for k in 1:42
+        Θ[k] = sum(x .* q .* e[k, :]') / sum(x .* q)
+    end
+
+    # Calculate s
+    s = zeros(42, 1)
+    for k in 1:42
+        s[k] = sum(Θ .* τ[:, k])
+    end
+
+    # Calculate L and J
+    L = r ./ sum(r .* x)
+    J = q ./ sum(q .* x)
+
+    # Calculate the natural log of the cumulative and residual activity coefficients
+    γ_C = 1 .- J .+ log.(J) .- 5 .* q .* (1 .- (J ./ L) .+ log.(J ./ L))
+    γ_R = zeros(1, size(ν, 2))
+    for i in 1:size(ν, 2)
+        γ_R[i] = q[i] * (1 - sum(Θ .* β[:, i] ./ s .- e[:, i] .* log.(β[:, i] ./ s)))
+    end
+	
+	# Return the activity coefficients
+    γ = exp.(γ_C .+ γ_R)
+
+end # function
 
 end # module