- How do I specify the position of a radical in the furane molecule (alpha C or beta C) in the Gaussian 09 input file?
- When we calculate the bond dissociation enthalpy (deltaH) of the bicycle molecule as 2,2' bifurane, is it correct to use the ground or transition state of this molecule as reactant?
- Is it necessary to reoptimize the product radical, or is SP sufficient?
Bond energy (and other thermodynamic quantities) are defined with the molecule as the initial state and the dissociated species as the final state. So, the bond energy of A-B is the energy difference EA + EB - EAB. To get delta-H, you must do a vibrational analysis of each species so that you have the (temperature-dependent) enthalpy corrections for each. See the Gaussian white paper on thermochemistry calculations: https://gaussian.com/thermo/
The bond cleavage enthalpy can be found in GAUSSIAN as the sum of the total enthalpy of radicals in the ground state at 298 K minus the total enthalpy of the compound at 298 K. All are optimized
As stated by others, usually it is as simple as doing a vibration analysis of the reactants and products and comparing the energy. But there is one thing you should account for: Basis Set Superposition Error (BSSE), in simple terms, it happens when the basis set you are using is not large enough to accurately describe the system, so some atoms from one molecule may "borrow" basis sets from the other and make your results meaningless.
A quick search will lead you to some papers explaining BSSE and how to overcome it, gaussian software even has a way around it using the counterpoise correction https://gaussian.com/counterpoise/. So, please, account for it whenever you do calculations that results in more than one fragment.
BSSE really is. But the error (deviation) in calculating of the total electronic energy is always greater than this error. Therefore, firstly, it is necessary to increase the basis set in order to reduce the error of the total electronic energy, but at the same time the BSSE also decreases
Vladimir Turovtsev You are right, the first step should always be making sure that you are using sufficiently large basis sets. As most chemists are using DFT as a "black-box" method, some considerations like BSSE, basis set completeness and the accurate option of the functional are just forgotten and should always be remembered.
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