  In the presence of a nonlocal potential in molecular device systems,
generally the charge conservation cannot be satisfied, and in literatures the
modifications of the conventional definition of current were given to solve
this problem. We demonstrate that, however, the nonconservation is not due to
the invalidation of the conventional definition of current, but originates
respectively from the improper approximations to electron-electron interactions
and the inappropriate definition of current using pseudo wave functions in
pseudopotential implementations. In this work, we propose a nonlocal-potential
formulation of the interactions to fulfill the charge conservation and also
give a discussion about the calculation of current when the pseudopotential is
involved. As an example of application of our formulation, we further present
the calculated results of a double-barrier model.
