  The entanglement entropy of a subsystem of a quantum system is expressed, in
the replica approach, through analytic continuation with respect to n of the
trace of the n-th power of the reduced density matrix. This trace can be
thought of as the vacuum expectation value of a suitable observable in a system
made with n independent copies of the original system. We use this property to
numerically evaluate it in some two-dimensional critical systems, where it can
be compared with the results of Calabrese and Cardy, who wrote the same
quantity in terms of correlation functions of twist fields of a conformal field
theory. Although the two calculations match perfectly even in finite systems
when the analyzed subsystem consists of a single interval, they disagree
whenever the subsystem is composed of more than one connected part. The reasons
of this disagreement are explained.
