  We revisit the description of the low-energy singlet sector of the spin-1/2
Heisenberg antiferromagnet on kagome in terms of an effective quantum dimer
model. With the help of exact diagonalizations of appropriate finite-size
clusters, we show that the embedding of a given process in its kagome
environment leads to dramatic modifications of the amplitudes of the elementary
loop processes, an effect not accessible to the standard approach based on the
truncation of the Hamiltonian to the nearest-neighbour valence-bond basis. The
resulting parameters are consistent with a Z$_2$ spin liquid rather than with a
valence-bond crystal, in agreement with the last density matrix renormalization
group results.
