[petsc-users] Question regarding the EPSSetDimensions routine

[baikadi pranay]

Thank you Jose for the reply.

If I set PETSC_DEFAULT_INTEGER for ncv as suggested in the EPSSetDimensions
documentation, I am still running into the same problem. Also, could you
elaborate on what you mean by checking convergence with a monitor. Do you
mean comparing the eigenvalues for ith and (i+1)th iterations and plotting
the difference to see convergence?

Sincerely,
Pranay.
ᐧ

On Thu, Feb 13, 2020 at 1:54 PM Jose E. Roman <jroman at dsic.upv.es> wrote:

> For nev=10 you are using a subspace of size 20. This may be too small.
> Check convergence with a monitor and increase ncv if necessary.
>
> Jose
>
> El 13 feb 2020, a las 21:25, baikadi pranay <pranayreddy865 at gmail.com>
> escribió:
>
> 
> Hello PETSc Users,
>
> I am trying to find the lowest 'n' eigenvalues of a hermitian eigenvalue
> problem. The size of the operator matrix (hamiltonian in my case) is
> dependent on the mesh spacing provided by the user (which is expected).
> However I have the following issue:
>
> The number of eigenvalues given by the solver is not consistent with what
> is given as input in the EPSSetDimensions routine. For example, for a
> 12000x12000 matrix, the solver gives 20 correct eigenvalues if nev=20, but
> fails to give any eigenvalue if nev=10.
>
> I am using the following lines of code to solve the problem:
>
>
> *call EPSCreate(PETSC_COMM_WORLD,eps,ierr)*
>
>
>
> *call EPSSetOperators(eps,ham,PETSC_NULL_MAT,ierr)call
> EPSSetProblemType(eps,EPS_HEP,ierr)call
> EPSSetWhichEigenpairs(eps,EPS_SMALLEST_MAGNITUDE,ierr)call
> EPSSetDimensions(eps,n_sub,n_sub*2,PETSC_DEFAULT_INTEGER,ierr)*
>
>
> *call EPSSetTolerances(eps,1D-10,5000,ierr)call EPSSolve(eps,ierr)*
>
> After the EPSSolve, I am calling EPSGetEigenPair and other relevant
> routines to get the eigenvector and eigenvalues.
>
> Any lead as to how to solve this problem would be greatly helpful to us.
> Please let me know if I need to provide any further information.
>
> Thank you for your time.
>
> Sincerely,
> Pranay.
>
>
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