  In our previous publication we have mistakenly claimed that the applicability
of the Hellmann-Feynman theorem in fixed-node quantum Monte Carlo calculations
is not subject to the manner how the nodal boundary depends on an external
parameter. This statement is not correct in general, except where the
Hellmann-Feynman force is calculated for a nodal boundary which coincides with
that of the unconstrained exact eigenfunction. We point out the error in our
arguments and present an explicit expression for the correction term which
supplements the Hellmann-Feynman force. This term can be interpreted as the
linear response of the energy with respect to variations of the the nodal
region.
