No. to Converge Before Shift Allowed (0 - Not Used) - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

Specifies the shifting strategy for the eigen problem to be solved as described below.

For a value of 0, CAESAR II selects an estimated optimal shifting strategy. Improving the convergence characteristics increases the speed of the eigensolution. The convergence rate for the lowest eigenpair in the subspace is inversely proportional to w1/w2, where w1 is the lowest eigenvalue in the current subspace and w2 is the next lowest eigenvalue in the current subspace. A slow convergence rate is represented by an eigenvalue ratio of one, and a fast convergence rate is represented by an eigenvalue ratio of zero. The shift is employed to get the convergence rate as close to zero as possible. The cost of each shift is one decomposition of the system set of equations. The typical shift value is equal to the last computed eigenvalue plus 90 percent of the difference between this value and the lowest estimated nonconverged eigenvalue in the subspace. As w1 shifts closer to zero, the ratio w1/w2 becomes increasingly smaller and the convergence rate increases. When eigenvalues are very closely spaced, shifting can result in eigenvalues being lost (as checked by the Sturm sequence check).

A large value entered for this parameter effectively disables shifting so that no eigenvalues are missed, but the solution takes longer to run. When the system to be analyzed is very large, shifting the set of equations can be very time consuming. In these cases, set the value between 4 and 8.