Dear Pratima Shukla ,

With that kind of calculation, what you are doing is optimizing the excited-state geometry, in particular you are optimizing the first excited state (S1, the root=1 keyword). At the end of that calculation, once you have the optimized structure, you will be able to take the vertical energy difference between the S1 and the ground state as the emission energy.

One important aspect that will reduce the computational cost. TDDFT states do not interact with each other, which basically means that your S1 state will always be the same regardless if you have calculated just that one, or five states, or a hundred... But the lesser amount of states, the lower computing time, so I recommend you to do a minimal adjustment: td=(singlets,nstates=1,root=1). With that, you will only compute the S1 excited state, which is the only one you are interested in (assuming there is no state flipping outside the Franck-Condon region, but for that you will need to optimize other excited states, so let's assume you are not interested in looking for that at this moment).

How much time will it take? Difficult to estimate, but having 74 atoms... be patient, it WILL take time.

Hope you find it helpful

Dear Pratima, please take a look at Gaussian's website

https://gaussian.com/%20scrf/

Here the emission from an excited state is explained in detail. You can text me if you have a doubt in this challeging calculation.

Edit: By the way, go to the "Examples" tab.

Best, Pablo