The principles and structure of an LCAO-MO ab-initio computer program which recalculates all twoelectron integrals needed in each SCF iteration are formulated and discussed. This approach-termed "direct SCF"-is found to be particularly efficient for calculations on very large systems, and also for calculations on small and medium-sized molecules with modern minicomputers. The time requirements for a number of sample calculations are listed, and the distribution of two-electron integrals according to magnitude is investigated for model systems.easily accessible at low cost. At present, the price/performance ratio for CPU and memory appears to decrease much faster than that for 1/0 operations and use of peripheral devices. It has sometimes been pointed out4 that a favorable I/O-to-cPU price ratio would make i t more economical to recompute integrals in every iteration than to store and retrieve these.
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