  The static current operator leads to definitional zero frequency divergence
and unphysical results in studying nonlinear optical susceptibilities of
polymers. A well-defined dipole-dipole correlation is superior to the
complicated current-current correlation to solve this problem. As illustrative
examples, optical susceptibilities under both SSH and TLM models of
trans-(CH)_x are studied. New analytical results are obtained. The reasons of
previous improper results are analyzed.
