I have optimized my organic system in the ground state using DFT method followed by TD-DFT energy calculation based on the optimized geometry. That is, the TD-DFT job does not contain an OPT command but "#p td=(nstates=10) rb3lyp/6-311++g(d,p) scrf=(solvent=ethylethanoate) geom=connectivity". So my question is whether this OPT command is mandatory in the TD-DFT job card, or not...? Also, how do we make a reasonable 'nstates' in the input line...? [By default it is 6, but we can increase this to some higher values, but how we have to know the optimum value for an organic system with 35 number of atoms...?]
Adding "opt" keyword would lead to optimization of excited state. Excited state optimization is required when you are working with excited state phenomenon such as fluorescence. Since absorption is related to transition to Frank-Condon excited state you should not add "opt" with "td".
Regarding what should be "nstates" it is dependent on your choice of energy range and experimental absorption spectrum. I think "nstates=10" should be enough.
Please have a look at these links
http://gaussian.com/faq4/
http://www.grs-sim.de/cms/upload/Carloni/Tutorials/FMCP/Exercise_acrolein_UV.pdf
https://www.researchgate.net/post/How_can_we_calculate_the_emission_spectrum_of_a_molecule_using_Gaussian/2
https://www.researchgate.net/post/How_can_I_obtain_the_UV-VIS_Spectrum_using_Gaussian_09
https://www.researchgate.net/post/How_can_I_calculate_a_UV-Vis_Spectrum_using_Gaussian_03
You have first to optimise the structure in the GS then use this optimized geometry in single point TD-DFT. You can choose to increase the number of states say by 10 rather than the default, in order not to miss an excited state if you use fewer states. Sometimes it is trial and error.
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