I am using ARPACK's zndrv1 to diagonalise a matrix (the context is quantum chemistry). While all vectors have a norm 1, as expected, vectors corresponding to degenerate eigenstates aren't always orthogonal (often their inner product will be roughly 0.01).
Are there any parameters I can adjust to improve orthonormality (even if it means longer compute time)?
I understand there are alternative libraries and routines out there. I am specifically curious about ARPACK.