How to account for Spin Contamination

What method in Q-Chem is advised for dealing with accounting for spin contamination in energy in large (520 atom, 2,640 electron, 2610 shells and 5694 basis functions) system?

It is not “accounting” so much as “reporting”. For a system of that size there is not much else that one can do, although ROKS is one option, and for UKS calculations not all density functionals are equally susceptible to spin contamination. If you are trying to examine an open-shell singlet state, there is the option to use approximate spin projection; see the OPSING $rem variable in the manual.

John, thanks for the reply.

I’m in the process of doing all the calculations with ROHF and ROKS.

I’m trying to determine the binding energies of DNA segments when separated from the original oligomer. I.e. I am fracturing the DNA and trying to determine the binding energy of the two segments when in the full oligomer.

To clarify, I start with DNA oligomer that I’ve labeled with two parts, DNA_total = DNA_LHS and DNA_RHS. I perform separate UHF/ UKS calculations on DNA_total, DNA_LHS and DNA_RHS. DNA_LHS and DNA_RHS are triplets, DNA_total is a singlet. The calculations are carried out as a standard BSSE, for DNA_LHS and DNA_RHS. The DFT calculations use the wB97M-V functional.

The DNA_total singlet has an <S^2> of 1.33/ 2.04 (UHF/UKS). DNA_LHS <S^2> is 2.12/2.0 (UHF/UKS) and DNA_RHS <S^2> is 2.07/3.02 (UHF/UKS).

Is there any literature that would help me understand the effect spin contamination has on these binding energies?

Thanks again for your help.