No. of Iterations Per Shift (0 - Pgm computed) - CAESAR II - Help

CAESAR II Users Guide

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CAESAR II
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CAESAR II Version
12

Specifies the number of subspace iterations per shift as described below.

For a value of 0, CAESAR II calculates an estimated optimal number of iterations. This parameter and % of Iterations Per Shift Before Orthogonalization control solution shifting by limiting the number of Gram-Schmidt orthogonalizations. Trying to limit this number is very dangerous for small subspace problems, but less dangerous when the subspace size is large, at around 10-20 percent of the total number of eigenpairs required.

Gram-Schmidt orthogonalization is by default performed once during each subspace iteration. The orthogonalization assures that the eigenvector subspace does not converge to an already found eigenpair. Many repeated eigenpairs calculations can appreciably slow down the extraction of the highest eigenpairs. Proper setting of these two parameters limits the orthogonalization in the eigensolution, such as to every second, third, or fourth iteration, and increases the solution speed. The subspace may still converge to earlier eigenpairs during subsequent non-orthogonalized subspace iteration passes.

Use caution when setting these parameters. Select Y as the value for Force Orthogonalization After Convergence (Y/N) if the frequency of orthogonalization is slowed.