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 that behaves as the secular term in perturbation theory. © 2012 Wiley Periodicals, Inc.