Chapter 2 Explicitly Correlated Approaches for Electronic Structure Computations
This chapter provides an overview of some fundamental concepts and recent developments, which make the chemical applications of explicitly correlated electronic structure methods feasible. The essential difference between conventional electronic structure methods and their explicitly correlated counterparts is in the functional form of the two-particle basis set. The conventional methods use the antisymmetrized products of one-particle orbitals to construct two-electron and higher-order basis sets. The explicitly correlated methods also add basis functions, which include explicit dependence on the distances between two electrons. The judicious use of the interelectronic distances in the n-electron basis greatly reduces the basis set error in the wave function computation of electronic structure. Recent progress in the area extends the applicability of explicitly correlated methods to molecules with tens and hundreds of atoms, all with relatively modest (double- and triple-zeta) basis sets. The future is certain to bring more development in this exciting area of research and in complementary subjects.