In Multiwfn there are two ways of calculating NLO quantities:
(1) Perform "polar" task in Gaussian, and use Multiwfn to extract data from its output file. See Section 4.200.7 of Multiwfn manual for example. This way corresponds to derivative method.
(2) Perform TDDFT task in Gaussian, and then use Multiwfn to calculate NLO data in terms of sum-over-states (SOS) based on excited state information, see Section 4.200.8 of Multiwfn maunal for example.

The first way doesn't need to explicitly calculate excited states.

Dear Prof. Tian Lu

Is the task for NLO analysis only in terms of the ground state (DFT) OR excited state (TDDFT)? When viewed in the Multiwfn module, the keyword used is POLAR in the ground state (DFT).

#NLO= nonlinear optic

Cheers,

Mono