Does anyone have experience estimating formation enthalpy or Atomization energy of molecules using Gaussian? I am trying to calculate this parameter for simple molecules like H2O and NH3, however, there is a significant error even with decent theory level. I am wondering how accurate this could be done? Is there any specific theory/method performing better than others? ( currently, I am using B3LYP/6-311++G (2d,2p) . I do not have any QM background and any thought would be appreciated.
Here are some numbers I am getting from Gaussian, compared to the JANAF table values.
Molecule: Gaussian JANAF
H2O : 1174 KJ --------- 917 KJ
NH3 : 1432 ------------ 1158
N2 : 1467 ----------------- 941
H2: 434 ------------------ 432
O2: 870 -------------- 493
Javad
In my experience composite method for example G3MP2, CBS-QB3, CBS-APNO etc. is very accurate for Enthalpy calculation.
With G3 compound method, I am getting value of 878.7 KJ for H2O which is still far less than value of 917 KJ reported in JANAF table !
Dear Javad,
It is very difficult to predict atomization energies with a DFT methods. For high accuracy coupled cluster is a requirement and, in order to save time, complete basis set extrapolation is another excellent option.
Even at this level, an important number of error arises from the theory and different correction and approximation must be done.
In other to perform the calculations only W1 and W2 theories are capable for an accuracy in the range of kJ/mol.
At this point, I invite you to read a very interesting (maybe essential) book and then, to apply the knowledge you will find there.1
I hope it helps you,
Best regards,
Joaquim Rius
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