Dear Manal Alwahsh ,

Use td=(nstates=1,root=1) pop=nbo density=current

• With root=1 you tell Gaussian that you are interested in the properties of the first excited state. This is what you also use to optimize that state, get its vibrational frequencies...
• With density=current you request that the post-SCF properties (like the dipole moment or the NBO analysis) are calculated using the specified density and not the ground-state one

As an addition, you should include polarization functions to get a reliable result, so at least use the 6-311G(d) basis set.

Hope you find it helpful

Dear Enrique Manuel Arpa Thank you for your helpful answer.

I use 6-311++g as a basis set which gives a reliable results .

if I use the optimized geometry of the first excited state as an input geometry to find the frequency or the wave function,

example: #p rb3lyp/6-311++g output-wfn

#P frq rb3lyp/6-311++g

would this strategy give me the same results as what you suggest ?

Dear Manal Alwahsh ,

I disagree, the basis set you just mentioned is unbalanced and will yield unreliable results. Including diffuse functions without polarization ones will heavily underestimate short-range interactions (i.e. chemical bonds) in favor of long-range effects, which is meaningless. At a bare minimum use 6-311+G(d) if you want to use a triple-zeta basis set with diffuse functions.

Now, td=(nstates=1,root=1) can be used for excited-state geometry optimizations, vibrational frequencies, population analyses… but not to obtain a wave function. output=wfn (or wfx) will still refer to the ground state.