I want to calculate the optimised structure for a diradical in the ground state using B3lyp, how would the input for it look like interns of the method like restrictEd or unrestricted, and the multiplicity and charge.
Thanks
Dear Mustafa Mahmoud,
For this purpose an input for Gaussian can be :
> #P UB3LYP 6-311++G** {choose your basis set} OPT POP=FULL GFINPUT IOP(6/7=3) FREQ
>
> put_a_name
>
> 0 1 (or 0 3 if triplet)
> {coordinates of your molecule}
could give you what you want. 0 is for the charge of you molecule and 1 (or 3) is its multiplicity.
What is the POP=FULL GFINPUT IOP(6/7=3) for, can it just be opt freq?
IOP(6/7=3) or pop=full means outputting more information into the log file. That is probably not what you need.
Diradical can be either singlet or triplet. Thus you should first make a judgement that which state is the ground state. If you are not sure, try both of them, and compare the energies for their optimized structure.
Triplet state is easy to handle. When calculating the singlet state, you should be careful, since it is highly possible to take an open-shell singlet (OSS) state. I.e. if you just write "opt freq b3lyp/6-31G(d)" and set the spin multiplicity as 1, the program will think all electrons are paired, and the restricted calculation is performed. However, for singlet diradical, the electrons are not totally paired. Thus you should use the guess=mix keywork (and don't forget to set unrestricted calculation). E.g.: opt freq guess=mix ub3lyp/6-31G(d).
For some difficult case, it is not enough to just use the guess=mix, since sometimes the program will give an unstable wavefunction, which is not the real wavefunction. So the most robust procedure is to run a stability calculation first:
1. stable=opt b3lyp/6-31G(d) (it means that check the stability of wavefunction (for example, if the RHF result is wrong, it will tell you and then find the real wavefunction))
2. ub3lyp/6-31G(d) guess=read opt freq (read the wavefunction generated by the previous step and run the following calculations)
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