INTEGRATED INTEGRAL EVALUATION

Integral-direct computational schemes have become standard techniques to avoid the integral-storage bottlenecks in ab initio electronic stru cture methods. In the integral-direct approach, the electron-repulsion integrals over Cartesian Gaussian basis functions are computed and pr ocessed on the fly whenever they are needed. The present work proceeds one step further. It inserts the integral evaluation into the integra l processing, i.e., into the four-index transformation. In this way th e integral transformation can start before the evaluation of the integ rals has completed. This concept of ''integrated integral evaluation'' has been studied in the framework of the second-order Moller-Plesset perturbation theory (MP2) approach which employs explicitly correlated n-electron functions which depend linearly on the interelectronic coo rdinates r(ij) (the MP2-R12 method). The new algorithm is particularly effective for the MP2-R12 method if large ''family'' basis sets with high l-value and low contraction depth are used.