The use of first and second derivatives in optical model parameter searches |
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Authors: | DH Gloeckner MH Macfarlane Steven C Pieper |
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Affiliation: | Rutgers University, New Brunswick, New Jersey 08903, USA;Argonne National Laboratory, Argonne, Illinois 60439, USA |
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Abstract: | This paper compares the efficiencies of standard minimization procedures in carrying out nuclear optical-model fits. It is shown that first-order perturbation theory permits computation of the gradient of x2 more than five times as fast as is possible by difference methods. A linear approximation to the second-derivative matrix in terms of first derivatives of residuals is found to be very accurate in the neighborhood of minima; it provides a way of introducing second-derivative information that is significantly superior to the use of variable-metric algorithms. The resulting restricted-step Gauss-Newton procedures are shown to be about five times as fast as direct-search methods. The use of the methods of pseudo-inverses to “freeze” linear combinations of parameters poorly determined by the data is discussed. |
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