  A fundamental quantity in 1+1 dimensional quantum field theories is
Zamolodchikov's c-function. A function of a renormalization group distance
parameter r that interpolates between UV and IR fixed points, its value is
usually interpreted as a measure of the number of degrees of freedom of a model
at a particular energy scale. The c-theorem establishes that c(r) is a
monotonically decreasing function of r and that its derivative may only vanish
at quantum critical points. At those points c(r) becomes the central charge of
the conformal field theory which describes the critical point. In this letter
we argue that a different function proposed by Calabrese and Cardy, defined in
terms of the two-point function of a branch point twist field and the trace of
the stress-energy tensor, has exactly the same qualitative features as c(r).
