| This work extends the development and application of local correlation, as pioneered by Pulay and Saebo, within the multireference singles and doubles configuration interaction method (LMRSDCI) in the Carter group. Local correlation techniques are useful in reducing the cost of electronic structure calculations by exploiting the short-ranged nature of electron correlation and hence leaving out insignificant pieces of the wave function based on a spatial criterion. In our original implementation, the occupied orbitals were localized by unitary transformations while the virtual subspace was spanned by non-orthogonal projected atomic orbitals (PAOs). By incorporating numerical screening techniques along with local truncation schemes, we were able to achieve an overall linear scaling method, reduced from its conventional very expensive O(N6) scaling, while retaining chemical accuracy. Subsequently, we carried out further optimizations to our LMRSDCI implementation by replacing the PAOs with localized orthogonal virtual orbitals along with restructuring the rate-limiting step within the diagonalization of the CI Hamiltonian matrix. In addition, we adopted alternative representations of the conventional two-electron integrals in the form of Cholesky vectors and density-fitted electron integrals. This switch in representation imparts flexibility towards the processing of the two-electron integrals, which is crucial to our restructuring efforts. While the evaluation of the Cholesky vectors and density-fitted integrals is not linear scaling, the overall computation cost of the calculation is lowered by greatly reducing the scaling prefactor. Hence, we were able to treat molecules containing as many as 50 heavy atoms with this accurate correlation wave function technique. Finally, we extended our LMRSDCI implementation from only calculating ground electronic states to be able to calculate valence excited states in large molecules. |
