The integral-direct coupled cluster singles and doubles model

An efficient and highly vectorized implementation of the coupled cluster singles and doubles ~CCSD! model using a direct atomic integral technique is presented. The minimal number of n processes has been implemented for the most time consuming terms and point group symmetry is used to further reduce operation counts and memory requirements. The significantly increased application range of the CCSD method is illustrated with sample calculations on several systems with more than 500 basis functions. Furthermore, we present the basic trends of an open ended algorithm and discuss the use of integral prescreening. © 1996 American Institute of Physics. @S0021-9606~96!00310-0#