Minimal operation-count method,

In the time-consuming step of the A/ -resoIution method, we did not exploit the separation of excitation operators into alpha and beta spin parts as in (11.7.10). We shall now consider a different method that more fully exploits the separation into the alpha and beta spin spaces. This algorithm is known as the minimal operation-count (MOC) method [4] since it yields an operation count that, to leading orders in the numbers of electrons and orbitals, is identical to the theoretical minimum (11.7.16). 

As demonstrated by the Pople-Hehre method it is possible to achieve considerable reduction in the computational expense of contracted ERIs if large parts of the integral manipulation are performed after the contraction step. The McMurchie-Davidson and the Obara-Saika methods utilization of the transfer equation (17) to minimize the operation count has been shown. This idea can, however, be employed to the extent that all manipulations are performed on fully or partially contracted integrals. Recently a number of methods have been presented along those lines.The method of Gill and Pople will be used as an example of the approach because it is currently one of the most commonly used integral methods. Note the concept of early contraction, however, applies to any of the methods presented in the chapter. 

The measurements were performed on three detectors. Two detectors, a Ge(Li) and a p-type, were connected to PGT 386 amplifiers. One of the amplifiers operated with a shaping time of 4 ps and the base-line restorer threshold set to VAR. With the second detector connected to the amplifier operating at 3 ps two sets of measurements were made one with the base-line restorer threshold set to AUTO and the other with the base-line restorer threshold set to VAR. The thresholds of the base-line restorers set to VAR were adjusted manually to minimize the tails of the pulser peak at low count rates. The third detector was a low-energy detector connected to an OR-TEC 573 amplifier operating with a shaping time of 6 ps and in the Gaussian mode. This amplifier was equipped with an automatic base-line restorer. The third detector was used in order to see how different amplifiers influence the performance of the pulser peak method. 

This method of measuring environmental tritium requires neither the large capital costs nor the large operating expenses of the enrichment techniques. While counting times may be long, the sample requires minimal preparation. 

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