[petsc-users] SLEPc: Convergence Problems

[Christopher Pierce]

Hello All,

As there isn't a SLEPc specific list, it was recommended that I bring my
question here.  I am using SLEPc to solve a generalized eigenvalue
problem generated as part of the Finite Element Method, but am having
difficulty getting the diagonalizer to converge.  I am worried that the
method used to set boundary conditions in the matrix is creating the
problem and am looking for other people's input.

In order to set the boundary conditions, I find the list of IDs that
should be zero in the resulting eigenvectors and then use
MatZeroRowsColumns to zero the rows and columns and in the matrix A
insert a large value such as 1E10 on each diagonal element that was
zeroed and likewise for the B matrix except with the value 1.0.  That
way the eigenvalues resulting from those solutions are on the order of
1E10 and are outside of the region of interest for my problem.

When I tried to diagonal the matrices I could only get converged
solutions from the rqcg method which I have found to not scale well with
my problem.  When using any other method, the approximate error of the
eigenpairs hovers around 1E00 and 1E01 until it reaches the max number
of iterations.  Could having so many identical eigenvalues (~1,000) in
the spectrum be causing this to happen even if they are far outside of
the range of interest?

Thank,

Chris Pierce
WPI Center for Computation Nano-Science


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