An expression has been derived for the diffusion coefficient in terms of the interatomic potential, the pair distribution function and the selfcorrelation function in a liquid. By making use of an earlier result for the time-dependent pair distribution function, which includes correlation in the relative motion of two atoms, the expression has been made applicable to any type of interatomic core potential. In particular it will be useful in a liquid metal where a relatively soft core is a generally accepted feature of the effective interionic potential.