The treatment of vacuum in semi-periodic post-HF methods

When a system has semi-periodic boundary condition, caution is needed to remove the interaction between the overall potential and the charge density in the neighboring boxes. For neutral systems, owing to the locality of one-body potential, a common trick in DFT calculations is to use a finite space of vacuum to mimic the free boundary. When moving to the periodic post-HF methods, the effectiveness of this treatment was yet verified. It was known that the finite-size correction to HF needs to be carefully handled in the periodic coupled cluster methods [1]. When the same treatment was applied to low-dimension systems, numerical uncertainty or slow convergence were observed. We compared the effects of vacuum treatments in periodic-MP2, periodic-CCSD calculations. We observed that the finite-size correction at HF level has different effects on different post-HF methods. In this talk, we will analyze the reasons that cause the numerical issues in semi-periodic post-HF methods. For different periodic post-HF methods, different solutions will be presented.

[1] J. McClain, Q. Sun, G. K. Chan, and T. C. Berkelbach, J. Chem. Theory Comput., 13, 1209-1218 (2017)