Is Argus lab accurate?

I was trying to calculate the resonance energy of benzene by using [Argus Lab](http://www.arguslab.com/arguslab.com/ArgusLab.html). These values are weird and very different from the experimental data. I have two questions regarding them.


1. What is the difference between Final SCF Energy and heat of formation?


When I tried to calculate the energy, It gave me two kinds of energy - Final SCF Energy and Heat of Formation. I know the definition of heat of formation. But what I found weird is that Heat of Formation of $\ce{H2}$ molecule was $\pu{-13.3564 kcal/mol}$. It should be zero instead.
2. Is there any mistake in following procedure:


a. I calculated the energy of cyclohexane($x$), cyclohexene($y$), and benzene ($z$).  

b. Energy change while converting cyclohexane to cyclohexene ($a$) $= y-x$  

c. Expected energy for benzene ($b$) $= x+3a$  

d. Resonance energy of benzene = $z-b$  

(Answer should be negative, implying that resonance energy is stabilizing in nature)


By using the SCF Energy data, I got resonance energy $\pu{415 Kcal/mol}$ and while using the heat of formation data I got $\pu{-41.8 Kcal/mol}$.


Here is the data (in $\pu{Kcal/mol}$) -:
$$
\begin{array}{|l|c|c|c|} \hline
\text{Energy type} & \mathrm x & \mathrm y & \mathrm z\\
\hline
\text{SCF Energy using PM3} & {-21556.7995} & {-20680.6471} & -18512.6189 \\
\text{Heat Of Formation} & {-37.43} & {-3.0101
} & 23.96 \\ \hline
\end{array}
$$


Is this a reliable software for calculating the resonance energy as there is a difference about $\pu{5 Kcal/mol}$ or I am doing some mistake by overlooking any concept?


