Mr/Miss/Mrs Bhattacharjee,

The topic is well described in teh Gaussian's manual.

Thank you mam.

Mr/Miss/Mrs Bhattacharjee,

In addition, you could pay attention to:

https://www.researchgate.net/post/How_to_calculate_optimized_structure_in_nth_excited_states_using_Gaussian

Dear Indranil,

You are asking about doing eom-ccsd? this is how to do so for two excited states and optimizing with respect to the 1st excited state, the states are singlets as am showing under the title after the 0 charge (neutral molecule) using 1, which is for singlet , for triplet states use 3 instead of 1

opt eomccsd=(nstate=2,root=1)

Title

0 1

But keep in your mind that eom-ccsd analytic gradients are only implemented in the recent Gaussian16 version, older versions will perform numerical gradients, which is extremely expensive