Really appreciated if any one can add some insight on this question I have. I want to optimize organic molecule using m062x level of theory and calculate energy values of that optimized structure using B3LYP. When I tried that, the energy optimizations were not converged. So I am wondering it might not possible to do frequency calculations and optimization using different level of theories ?
Dear Sudeera Kamburugamuwe ,
To answer your question, yes it is possible to do this. There are countless examples where the GGA of PBE is used to optimize the structure of a material and then hybrid functionals (like B3LYP) are used to calculate band structure. In principle, you can choose any level of theory you like.
With that being said, it may be the case that two different levels of theory give two different ground state structures. Therefore, if you use one level of theory to optimize the structure and another to calculate the frequencies, then some of the frequencies may be imaginary, i.e. displacing an atom away from its equilibrium position decrease the total energy.
My recommendation would be to optimize the structure using a coarser level of theory and then re-optimize the structure using B3LYP (if you can, of course).
I hope this was helpful. Please let me know if you have any further questions.
Thank you so much for the reply Robert Wexler . I am trying to optimize organic molecules using M062X level of theory and calculate frequencies using B3LYP. But most of the time I have problems in convergence of molecules. But when I try both opt and freq in same level of theory I dont have this problem and molecules are nicely converged. If you have any insight on this please let me know. Thanks again.
Sudeera Kamburugamuwe , could you clarify what you mean by "problems in convergence"?
What could be happening here is that the two levels of theory produce different optimized structures. Since frequency calculations involve small displacements from the optimized structure, the total energy should be increased. If you use B3LYP for frequency calculations of the M062X optimized structure, then the energy might go down if the B3LYP optimized structure is different and the displacement brings the structure closer to that. This will produce what are called imaginary frequencies.
Robert Wexler , Thank you very much for the explanation. Now I understand how two level of theories going to effect opt and freq calculations.
I meant by ''problems in convergence'' is - When I tried those two level of theories, most of the times Maximum displacement and RMS displacement were not converged. But when I tried same level of theory for both opt and frequency calculations I didn't have that problem.
Thank you.
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