However, the t 1 -dressed formalism of coupled-cluster theory initially lacked popularity due to the cost of transforming four-center, two-electron integrals every CC iteration. However, DePrince et al. have shown that when the density-fitting (DF)/resolution of the identity or Cholesky decomposition (CD) approximations of two-electron integrals are applied, the use of the t 1 -transformed formalism becomes worthwhile. DePrince used t 1 -dressed integrals in his implementation of DF-CCSD, while Schwilk et al. and Jiang et al. used the t 1 -transformed formalism in their implementations of local pair natural orbital based CCSD. In this formalism, the two-electron integrals are approximated as: