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Novel computation enables best-yet estimate of ground state of neutral helium. (News Briefs).

A NIST scientist, in collaboration with Indiana University, has computed the nonrelativistic energy for the ground [S.sup.1] state of neutral helium to be -2.9037 2437 7034 1195 9829 99 a.u. This represents the highest accuracy computation of this quantity to date. Comparisons with other calculations and an energy extrapolation yield an estimated uncertainty of [10.sup.-20].

Exact analytical solutions to the Schrodinger equation, which determines such quantities, are known only for atomic hydrogen and other equivalent two-body systems. Thus, solutions must be determined numerically. How this best calculation to date was accomplished is in an article submitted to the International Journal of Quantum Chemistry. To obtain this result, very large basis sets must be used. In this case, variational expansions of the wave function with 4648 terms were employed, leading to the need for very large computations. Such large expansions also lead to problems of linear dependence, which can only be remedied by using higher precision arithmetic than is provided by standard computer hardware. For this computation, 192 bit precision (roughly 48 decimal places) was necessary, and special coding was required to simulate hardware with this precision.

Parallel processing was also employed to speed the computation, as well as to provide access to enough memory to accommodate larger expansions. NIST's Scientific Computer Facility cluster of 16 PCs running Windows NT was utilized for parallel computation. Typical run times for a calculation of this size about are 8 hours on a single CPU, but only 30 min to 40 min on the parallel processing cluster.

This work employs a very novel wave function, namely, one consisting of at most a single [r.sup.12] raised to the first power combined with a conventional nonorthogonal configuration interaction basis. The researchers believe that this technique can be extended to multielectron systems. Work is in progress, for example, to see what uncertainties can be obtained for atomic lithium, which is estimated to require a 6000-fold increase in CPU requirements to reach the same level of precision, making the use of parallel programming techniques even more critical.

CONTACT: James Sims, (301) 975-2710; james.sims@nist.gov.
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Publication:Journal of Research of the National Institute of Standards and Technology
Article Type:Brief Article
Geographic Code:1USA
Date:Jan 1, 2002
Words:356
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