Simplified multireference coupled‐cluster methods : hybrid approaches with averaged coupled pair theories

Abstract

We define an approximation to the internally contracted multireference coupled‐cluster method with single and double excitations by a hybrid approach. The rationale is to treat the external pair energy contributions by the coupled‐cluster method, which provides accurate results for a large part of the correlation energy while being tractable as the involved pair cluster operators commute. For the internal and semi‐internal contributions, for which the coupled‐cluster part becomes involved due to non‐commuting operators, a linearized approach based on the coupled‐electron pair approximation (CEPA) is used. For the latter, the CEPA(0) method, the averaged coupled pair functional (ACPF), the averaged quadratic coupled‐cluster (AQCC) method, and the averaged CEPA method are tested. We test the methods concerning size consistency, potential energy curves for C2, N2, CN, and O3 and for the singlet‐triplet splitting of ortho‐, meta‐, and para‐benzynes. Our results show that AQCC provides the most accurate results and stable performance. The main drawback of the method is that it shows small violations of size consistency.