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sanya
sanya@photonics.ru

You are right, the problem of dissociation of diatomic molecules is not as simple as it may seem. The potential curves corresponding to dissociation into different products (say, A^. + B^. or A⁺ + B^- or something else as you described) may cross. That is, it may happen (and, most frequently, happens) that the potential curve corresponding to the lowest-energy minimum leads to high-lying products, while higher-lying minimum dissociates into lower-lying products. Moreover, the picture of the potential curves will depend on the method you use (RHF, UHF, MCSCF, CCSDT, etc.). Hence, there cannot be any standard methods or keywords.

“If we knew what we were doing, it would not be called research, would it?” (A. Einstein)

First of all, try to find and read the book by G. Herzberg, Molecular Spectra and Molecular Structure: Spectra of Diatomic Molecules. It is rather old but very useful.

On Wed Dec 8 '10 8:37pm, David G. wrote
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>Dear Firefly Users

>I want to calculate the D0 (Dissociation energy) of some diatomic molecules and the trivial procedure is calculating the energy of AB molecule and subtract it from the calculated-energy of A and B. If the AB has higher spin multiplicity than 1, it may make a question that what is the spin multiplicity of the dissociative products ? Are they all in their ground states as the free atoms or not ? the same question sometimes also may arise when the system has a closed shell. Certainly, the results of this trivial procedure depend on our selections and sometimes may cause wrong calculated-D0. Is there any standard method or tricky keyword in Firefly to overcome this problem or makes it easier to encounter ? I appreciate your help in advance.  
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>Sincerely Yours
>David G.
>JCU
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