Nice Question.

I am also having same question. If you find the answer, please let me know.

Rishu Khurana I am sharing the method to calculate the diradical character as I learned from Prof. Soichi Ito from IMS Japan:

"Natural orbital" mentioned here represents the eigenvectors of the electron density matrix. The values are obtained from the Gaussian program package using the keyword "pop=no." Here "no" means natural orbital. You may care that you have to choose an appropriate method to calculate the occupation numbers. If you set the calculation method such as, say, HF, it gives integer values 2 for occupied and 0 for virtual natural orbitals for the singlet ground state, by definition. You need the spin-unrestricted calculation, say, UHF., for the SINGLET state calculation in evaluating the occupation number of virtual orbitals. A Gaussian09 keyword "stable=opt" may be helpful to obtain a spin-symmetry broken wavefunction ( > 0), though you need to care about the result.

A keyword "guess=mix" is also helpful in the evaluation of > 0 broken symmetry wavefunction and non-zero occupation number of natural orbitals by using UHF (or, say, UB3LYP).

You can also use other electron-correlated wavefunctions such as CISD, CCSD, or CASSCF. Evaluate the occupation numbers for natural orbitals when you want to get a spin-adapted (=0) wavefunction. Hope it helps. Thanks

Please refer to this discussion:

https://www.researchgate.net/post/How-would-an-input-for-a-diradical-be-in-gaussian-g09