Can any one suggest me how to find Eigen Values for a very large matrics which is not positive definate?

In eigs>checkInputs/LUfactorAminusSigmaB at 1045 In eigs>checkInputs at 855 In eigs at 94 Warning: Matrix is close to singular or badly scaled. Results may be inaccurate. RCOND = 1.605094e-16.

Every time I execute [Y,S,convergence] = eigs(Xsub,20,'SM'); I get this error.I tried things available over the internet, but nothing is working. Xsub is a Laplacian matrix which should be positive definite, but its not.