The SCF calculation produces a density, *P*, and Fock
matrix, *F*. These, together with the one-electron matrix, *H*,
allow the total electronic energy to be calculated via

The total core-core repulsion energy is given by:

The addition of these two terms represents the energy released when the ionized atoms and valence electrons combine to form a molecule.

A more useful quantity is the
heat of formation of the compound from its elements in their
standard state. This is obtained when the energy required
to ionize the valence electrons of the atoms involved
(calculated using semiempirical parameters),
*E*_{isol}(*A*), and heat of formation for a gaseous
atom from its standard state,
*E*_{atom}(*A*), are added to the electronic
plus nuclear energy. This yields:

Δ*H*_{f }= (*E _{elect}*
+

or

Δ*H*_{f }= *E*_{tot}
- Σ_{A}*E _{isol}*(

This is the quantity which MOPAC calls the "Heat of Formation".
An alternative but equivalent definition of
ΔH_{f}_{ },
more
suited for comparison with experimental
ΔH_{f}, is:

"
ΔH_{f}_{ }is the calculated gas-phase heat of formation at 298K
of one mole of a compound from its elements in their standard state."

Things to note about this definition: unlike *ab initio* methods, which
yield the energy at 0*K*, semiempirical methods give
ΔH_{f}_{ }at 298*K*. This follows from the way in which semiempirical methods
are parameterized: the reference
ΔH_{f}_{ } are conventionally given at
298*K*. This means that semiempirical methods will reproduce
ΔH_{f} for 298*K*. Secondly, note that
ΔH_{f}_{ } are for gas-phase systems.
To calculate
ΔH_{f}_{ } in the liquid or solid phases, additional terms are
necessary.