Response to "Comment on 'A centroid molecular dynamics study of liquid para-hydrogen and ortho-deuterium'" [J. Chem. Phys. 122, 057101 (2005)] -: art. no. 057102

The issue of the proper classical limit for approximate quantum dynamical methods is discussed, especially as it applies to liquid para-hydrogen. The conclusion is that a logical dilemma arises, in which the quantum system is in one thermodynamic state but the classical system for the same volume is in a different thermodynamic state. This dilemma also points to the likelihood that a certain class of approximate quantum dynamical methods, in which the initial conditions are weighted by a quantum distribution function but the dynamics are then run with classical trajectories, will contain a systematic error in the dynamical results for such systems. © 2005 American Institute of Physics.