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sanya
sanya@photonics.ru

It's not a problem :)

The definition of the heat of formation (cited from Wikipedia) says:
The standard enthalpy of formation "standard heat of formation" of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states (the most stable form of the element at 1 bar of pressure and the specified temperature, usually 298.15 K or 25 degrees Celsius).
This is the value you obtain from NIST.

Therefore, you should consider the following reaction for LiH:

2LiH|_{solid, 298K} = 2Li|_{metal, 298K} + H₂|_{gas, 298K}

That is, ΔE_f(LiH) = 1/2(2*E(LiH) - E(H₂) - 2*E(Li)), where all E values are extrapolated to the standard conditions. Obviously, you cannot extrapolate the energy of Li atom to the energy of Li metal at 298K from the first principles. Therefore, you cannot compare your ab initio calculated value with the reference experimental data immediately.

On the other hand, semiempirical quantum chemistry methods are calibrated so that they directly give you heats of formation instead of absolute energies.

The value you obtained is just the bond dissociation energy under the assumption that the bond breaks homolytically (Li atom + H atom) and the molecular geometry used was equilibrium. This value can be compared with the experiment (search for "bond dissociation energy" or "bond dissociation enthalpy" for the molecule of interest).