What kind of keywords are need to be given for that type of calculation?
Which quantum chemistry program do you use? The quantum chemistry program ORCA is good choice to do the calculation you are looking for. In ORCA, use hybrid density functional (e.g. B3LYP) to obtain the broken-symmetry solution at the DFT level for your system using "%scf brokensym 1,1" with a final Ms=0 (for your di-radical case with one unpaired electron on each part of the system). Once you obtain the solution the program will give you the coupling constant (J) using three different method. Otherwise you can calculate it by hand using Yamaguchi formula (K. Yamaguchi, H. Fukui, and T. Fueno, Chem. Lett. 1986, 625–628 (1986)). The parameters needed can be obtained from the broken-symmetry solution.
Dear Bhaskar Sir,
Firstly, I like to thanks for your kind reply. I use the Gaussian 09 for the calculation. I like to know to do this type calculation in Gaussian, how input could be build or keyword to be give. Please, give me a reply sir.
Dear Snehasis,
One way to calculate coupling constant of diradical complexes is to first optimize the molecule and using this geometry do a broken symmetry calculation with guess=mix keyword. I am giving a sample keyword line.
# ub3lyp/6-311++g(d,p) geom=checkpoint guess=(read,mix)
Using energy difference of this broken symmetry solution and high spin state, you can easily calculate coupling constant using Yamaguchi formula.
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