I did a frequency calculation for acetaldehyde, and i want to know how to determine that the molecule is in global minima. I frequency calculated output file is attached.
If you are really looking for the global minimum, you cannot tell from frequencies alone; having all positive normal mode frequencies (excluding the 5-6 translational + rotational modes, which should be small but often have residual non-zero values) only tells you that you have found a local minimum (a minimum with respect to all small displacements of any atoms from their current positions). There is no algorithmically automatic way to be certain you have a global minimum, although for sufficiently simple molecules if you step through all possible bond lengths, angles, and dihedrals or, more reasonably, use a search method like Osawa's, you can be essentially certain. For more flexible molecules, it is often useful to either 1. use chemical intuition and check what you think are the likely conformers or 2. use a Monte Carlo or MD conformer generator and then optimize each of those leads. Either way, you must then make the assumption that the lowest-energy conformer you optimized is the global minimum.
Mr. Ranganath,
Please pay attention to:
https://www.researchgate.net/post/How_may_it_be_possible_to_describe_a_protonation_process_via_an_IRC_calculation_How_may_I_depict_the_Transition_State_of_the_reactive_system
Dear Saeed,
Yes that's correct, but the i want to know using frequency calculation how we are going to say a molecule is in global minima.
As a example in the 1st attached figure it's frequency calculation for CH3-CH3 and i just drew it in Avogadro and did a frequency calculation using GAMESS.
2nd figure, it's a frequency calculation for optimized CH3-CH3 using DFT
Observing these values how can we say which obtained the minima ?
Mr. Ranganath,
Your example is a textbook knowledge.
You could check your data within the framework of my attached analyses.
Conformational analysis by Gamess-US can be obtained, computing energy versus digedral [H-C-C-H] angle value.
Dear Yasashri Ranganath,
Look for the imaginary frequency, if you didn't encounter any, then the structure is at its global minima.
Dear Ivanova and Mohsin Yousuf,
Thank you very much for your support.
I am not familiar with GAMESS, but in general everything is clear cut after you plot the energy curve :)
If you are really looking for the global minimum, you cannot tell from frequencies alone; having all positive normal mode frequencies (excluding the 5-6 translational + rotational modes, which should be small but often have residual non-zero values) only tells you that you have found a local minimum (a minimum with respect to all small displacements of any atoms from their current positions). There is no algorithmically automatic way to be certain you have a global minimum, although for sufficiently simple molecules if you step through all possible bond lengths, angles, and dihedrals or, more reasonably, use a search method like Osawa's, you can be essentially certain. For more flexible molecules, it is often useful to either 1. use chemical intuition and check what you think are the likely conformers or 2. use a Monte Carlo or MD conformer generator and then optimize each of those leads. Either way, you must then make the assumption that the lowest-energy conformer you optimized is the global minimum.
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