# CCSD and CISD size consistency problem

I have performed a Gaussian16 calculation to explore the size-consistency problem for the CISD and CCSD methods. The system I am studying is the water dimer and the results are not what I expected at first. I have computed the energy of the dimer for both models (single point calculation) where the two monomers are 100 Å away and the energy of the and the energy of the monomer.

I wanted to compute the difference between those energies for both methods as two times the energy of the monomer and the energy of the dimer. If a method is size-consistent, the difference should be zero. These are my results:

$$\begin{array}{lrr} \hline \text{Quantity} & \text{CCSD} & \text{CISD} \\ \hline \text{Monomer energy}/\pu{Ha} & -75.012 & -75.012 \\ \text{Dimer energy}/\pu{Ha} & -150.030 & -150.026 \\ \text{Abs. difference}/\pu{Ha} & 0.004 & 0.002 \\ \hline \end{array}$$

I have been reading many articles and papers and what I expected is that CISD is not size consistent for a system of more than two electrons while CCSD is size-consistent.

However, the value of the difference is larger in the case of CCSD, what makes me think that it is telling me that CCSD is less size-consistent for my problem than CISD. Am I wrong? What do these values mean?

• When you write monomer energy, is it the true monomer energy, or did you use the "dimer" at 100 Angstrom distance? Apr 10 at 9:30
• It is the monomer energy, while the distance of the dimer is the energy of the dimer where the two monomers are 100 A away Apr 10 at 9:31
• I don't have the answer to your question, but you could opt to migrate this question to matter modelling SE. That site is specifically for computational modelling and there may be a better chance of an answer there. Apr 10 at 17:21
• Thank you for the suggestion. I have already posted the question and can be found here: mattermodeling.stackexchange.com/questions/4693/… Apr 11 at 8:19