<html><head><meta http-equiv="Content-Type" content="text/html charset=us-ascii"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Hi, <div class=""><br class=""></div><div class=""> I am using slepc to solve for the natural frequencies of a small-disturbance modal eigensolution of a plate with compressive stresses. </div><div class=""><br class=""></div><div class=""> M x = (1/omega^2) K(lambda) x</div><div class=""><br class=""></div><div class=""> lambda is the loading parameter. I first solve for the nonlinear static solution for a given load, which gives K (after linearization), which is used here. </div><div class=""><br class=""></div><div class=""> Note that I am solving for 1/omega^2, instead of omega^2, since the mass matrix may have zero entries related to some dofs, while the stiffness matrix of the constrained structure has no zero eigenvalues (except, perhaps in the nonlinear region at the bifurcation point). </div><div class=""><br class=""></div><div class=""> As lambda increases, the structure crosses into the post-buckled region, so a bifurcation point is crossed at some lambda. The fundamental natural frequency with this tangent stiffness matrix is expected to reduce to zero, and then increase after the bifurcation point. </div><div class=""><br class=""></div><div class=""> I am calling slepc without any command line options, except that I specify LU-decomposition as the linear solver. </div><div class=""><br class=""></div><div class=""> The challenge that I am facing is that I am able to get eigensolutions till the lowest natural frequency drops to 0, and then the solver quits with the following error message: </div><div class=""><br class=""></div><div class=""><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: --------------------- Error Message --------------------------------------------------------------</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: Error in external library</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: Error in Lapack xHSEQR 100</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: See <a href="http://www.mcs.anl.gov/petsc/documentation/faq.html" class="">http://www.mcs.anl.gov/petsc/documentation/faq.html</a> for trouble shooting.</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: Petsc Release Version 3.6.3, Dec, 03, 2015 </span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: /Users/manav/Documents/codes/MAST/build/opt/example_driver on a arch-darwin-cxx-opt named <a href="http://dhcp-90-243.hpc.msstate.edu" class="">Dhcp-90-243.HPC.MsState.Edu</a> by manav Sun Apr 3 13:31:08 2016</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: Configure options --prefix=/Users/manav/Documents/codes/numerical_lib/petsc/petsc-3.6.3/../ --CC=mpicc --CXX=mpicxx --FC=mpif90 --with-clanguage=c++ --with-fortran=0 --with-mpiexec=/opt/</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">local/bin/mpiexec --with-shared-libraries=1 --with-x=1 --with-x-dir=/opt/X11 --with-debugging=0 --with-lapack-lib=/usr/lib/liblapack.dylib --with-blas-lib=/usr/lib/libblas.dylib --download-superlu=yes -</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">-download-superlu_dist=yes --download-suitesparse=yes --download-mumps=yes --download-scalapack=yes --download-parmetis=yes --download-parmetis-shared=1 --download-metis=yes --download-metis-shared=1 --</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">download-hypre=yes --download-hypre-shared=1 --download-ml=yes --download-ml-shared=1 --download-sundials=yes --download-sundials-shared=1</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: #1 DSSolve_NHEP() line 622 in /Users/manav/Documents/codes/numerical_lib/slepc/slepc-3.6.2/src/sys/classes/ds/impls/nhep/dsnhep.c</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: #2 DSSolve() line 543 in /Users/manav/Documents/codes/numerical_lib/slepc/slepc-3.6.2/src/sys/classes/ds/interface/dsops.c</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: #3 EPSSolve_Arnoldi() line 124 in /Users/manav/Documents/codes/numerical_lib/slepc/slepc-3.6.2/src/eps/impls/krylov/arnoldi/arnoldi.c</span></div><div style="margin: 0px; font-size: 10px; line-height: normal; font-family: Monaco; color: rgb(245, 245, 245); background-color: rgb(0, 0, 0);" class=""><span style="font-variant-ligatures: no-common-ligatures" class="">[0]PETSC ERROR: #4 EPSSolve() line 101 in /Users/manav/Documents/codes/numerical_lib/slepc/slepc-3.6.2/src/eps/interface/epssolve.c</span></div></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""> Is this related to the loss of positive-definite nature of the stiffness matrix? Is there a way I can still get solution form slepc? </span></div><div class=""><span style="font-variant-ligatures: no-common-ligatures" class=""><br class=""></span></div><div class=""> I would greatly appreciate any advice. </div><div class=""><br class=""></div><div class="">Regards,</div><div class="">Manav</div><div class=""><br class=""></div></body></html>