we are doing calculation on gaussian and optimized our structure using b3lyp functional and also apllied electric field on it, after that we are performing td-dft calculation on that optimized structure but the level of theory is different it is CAM-B3LYP and also we are employing solvent model as well as the electric field with opt=z-matrix keyword. Now in place of single point energy calculation it is performing opimization, so would it effect my results if the ground state is optimized on different level of theory while the excited state would get optimized on different level of theory. kindly answer my query.
It is not strictly mandatory, and the literature reports several cases of adoption of different levels of theory for optimization and td calculations. Nevertheless, when changing the computational setup (functional/basis set/solvation, and so on), the electronic structure cannot be fully relaxed under both conditions. Indeed, small geometry variations deriving - for example - from solvation will induce often significant variations in the eigenvalues and even on their order (especially in the case of compounds with low-energy electronic transitions). Hence, I suggest adopting the same level of theory along all the series of calculations. In your case, it will not require a lot of work to start from the chk file of the b3lyp optimization and introduce the cam-b3lyp and the solvation, before calculating the vertical electron transitions by time-dependent DF theory.
Hello
According to the information I have, when we want to make calculations about the molecular structure, we must first optimize the structure. I have followed the same process in the work I have done.
Investigation of Structural and Optoelectronic Properties of Sc2O3 Nanoclusters: A DFT Study
Of course, according to the basic states and methods and the type of solvent, the amount of energy will be different, and it is usually the case that energy states are compared for one method and one basic state, and then the results are compared with another method and basic state. .
Why do you want to choose different levels of theory for ground and excited state?
In my opinion, it is better for you to define the topic precisely first and then read numerous articles and books in the desired field.
For example, people who have worked like you, then proceed with the calculations to save time and get the right results.
Massimiliano Arca Saeideh Hosseini sir i have done geometry optimization of my molecule at B3LYP/LANL2DZ/6-31G(d) level of theory and i m investigating the excited state properties of the molecule but at different level of theory i.e. TD-DFT CAM-B3LYP/LANL2DZ/6-31G(d) with IEF-PCM solvent model with THF solvent. Can i apply TD-DFT on above optimized molecule which is at B3LYP level or should i optimize molecule again at the level of theory that of TD-DFT. Would it effect my results? and also please give me a little insight about calculating the vertical excitation energy.
Thanks a lot for the answer .
Ravi Kumar
In my personal opinion, you may want to re-optimize the structure of your compound in the presence of the solvent (for example with scrf=solvent=tetrahydrofurane) starting from your already-optimised geometry (geom=allcheck guess=read chkbasis). After this step you can repeat the TD calculation. Most probably, you will find small but significant differences with the previous calculation. The energy transition provided by the TD keyword in Gaussian gives you the energy and oscillator strenght of each vertical excitation transition along with its composition in terms of one-electron excitations.
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