  We revisit the external source method for Kubo-transformed quantum
correlation functions recently proposed by Krishna and Voth. We derive an exact
formula and show that the Krishna-Voth formula can be derived as an
approximation of our formula. Some properties of this approximation are
clarified through a model calculation of the position autocorrelation function
for a one-dimensional harmonic oscillator. A key observation is that the
Krishna-Voth correlation function has a term which behaves as the secular term
in perturbation theory.
