Ruud et a/.164 have attempted to introduce an integral screening procedure into direct SCF calculations of the second hyperpolarizabilities of large molecules. The screening simulates the correlation effects that are implicit macroscopically in the introduction of a dielectric constant. The introduction [Pg.21]

Tlie combination of these effects means that the increase in computational time of a direct SCF calculation over a disk based method is less than initially expected. For a medium size SCF calculation which requires say 20 iterations, the increase in CPU time may only be a factor of 2 or 1 Owing to the more efficient screening, however, the direct [Pg.79]

The horizontal arrows in Figures 1 and 2 correspond to contraction steps and, in typical Direct SCF calculations using PRISM, these account for a significant fraction of the total CPU time (15% in the pentacene run described in Section 4.7). In electrostatic grid calculations, the fraction is even higher. It is therefore very important that they be executed as efficiently as possible. [Pg.183]

With the current impressive CPU and main memory capacity of relatively inexpensive desktop PC s, non-direct SCF ab initio calculations involving 300-400 basis functions can be practical. However, to run these kinds of calculation, 20 GBytes of hard disk space might be needed. Such big disk space is unlikely to be available on desktop PCs. A direct SCF calculation can eliminate the need for large disk storage. [Pg.266]

See also in sourсe #XX -- [ Pg.115 , Pg.265 ]

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