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Dominic P. Guaņa
organicchemistry_01@yahoo.com.ph

I have perfromed 1, but it gave me

E = -58.17 kcal/mol.

I have also performed number 4, I have used GAMESS (US) for this, but I guess it will give me the heat of formation using that theory level, and also it gave me a data from NIST database, 33.3kcal/mol.

Moreover, B3LYP/TZV(pd) calculation gave me a very different result for the Heat of Formation, it seems that it does not bode well with the established data. I dont know why. (All of the calculation has been succesfull and finished normaly)

Also thanxs for verifing my formula. And Thanxs in advance.


On Thu Oct 29 '09 3:07pm, Igor Polyakov wrote
---------------------------------------------
>Hello, Dominic!

>Frankly speaking it was kind of hard to understand your message, that is why i will post some general comments:
>1) Yes, you can determine the heat of formation of LiH from the reaction:
> Li+H=LiH and yes E(LiH)-(E(Li)+E(H))=HeatofFormation(LiH)
>For the E(LiH) you should use the LiH equilibrium geometry, for the E(Li)+E(H) you can do 2 separate calculations(i think in this case this is a preffered option) or make 1 calculation with a large distance between Li and H.
>2) You can compare accuracy of different methods but you must use size consistent methods, for example Hartree-Fock, coupled cluster, many-body perturbation theory (to any order), and full configuration interaction (CI)
>3)In order to compute the heat of formation more accurately you will need to compute hessian and do the thermochemical analysis for the given temperature.
>4)In order to save computer time and get the very accurate result for the energy calculation people designed several computational schemes such as G1, G2 and etc, for example:

>the G2(MP2) method involves the following steps: (copypasted from one of the QC books)
>(1) The geometry is optimized at the HF/6-31G(d) level and the vibrational frequencies
>are calculated.To correct for the known deficiencies at the HF level, these are
>scaled by 0.893 to produce zero point energies.
>(2) The geometry is re-optimized at the MP2/6-31G(d) level, which is used as the reference
>geometry.
>(3) An MP2/6-311+G(3df,2p) calculation is carried out, which automatically yields the
>corresponding HF energy.
>(4) The energy is calculated at the QCISD(T)/6-311G(d,p) level. This automatically
>generates the MP2 value as an intermediate result, and the difference between
>the QCISD(T) and MP2 energies is taken as an estimate of the higher order correlation
>energy. The G2 method (not G2(MP2)) performs additional MP4 calculations
>with larger basis sets to get a better estimate of the higher order correlation
>energy.
>(5) To correct for electron correlation beyond QCISD(T) and basis set limitations, an
>empirical correction is added to the total energy, ΔEemp = −0.00481Na − 0.00019Nb,
>where it is assumed that the number of b electrons is larger than or equal to the
>number of b electrons. The numerical constants are determined by fitting to the
>reference data. It should be noted that this correction makes the G2 methods nonsize
>extensive.

>I suppose, u get the general idea.

>Glad if my post helps,

>Best regards, Igor.
>
>
>On Wed Oct 28 '09 3:31pm, Dominic P. Guaņa wrote
>------------------------------------------------
>>i am computing the heat of formation by first runtyp=energy for the energy of individual atoms.... adding them up then subtract with the energy computed from the their most stable geometry. Optimization, hessian at B3LYP/TVZ (pd) level... i want to test the accuracy of the theory level i am using. I tried this with LiH,

>>This is my Formula, calculation at B3LYP/TZV (pd) level

>>Heat of Formation = E(LiH)-(E(Li)+E(H))

>>gives me -58.17 kcal/mol.

>>HELP with this. THanxs in advance, i dont know if my formula is correct or will give an accurate result for the accuracy of the Theory Level.

>>Thanxs in advance.