In reply to Siladittya Manna.

Yes. In the program the relative change is abs((lambda - lambdaOld)/lambda).

You say "the eigenvectors I get for the maximum eigenvalue" changes from iteration to iteration.
You should verify that the matrix is symmetric so that all eigenvalues are real. If the largest eigenvalue is complex, the power method is useless. Also, see the last section if the largest eigenvalue is negative.

Lastly, you say "the eigenVECTORS ...are of the order of 10^-6." I do not know what that means. Do you mean that the magnitude of the typical element is 1e-6? That makes sense. At each iteration the eigenvector is renormalized so that ||v||=1. Since the vector contains 150k elements, I would not be surprised if a typical element has magnitude of 1e-6.