  We present a variational MonteCarlo (VMC) and lattice regularized diffusion
MonteCarlo (LRDMC) study of the binding energy and dispersion curve of the
water dimer. As a variation ansatz we use the JAGP wave function, an
implementation of the resonating valence bond (RVB) idea. Actually one the aim
of the present work is to investigate how the bonding of two water molecules,
as a prototype of the hydrogen-bonded complexes, could be described within an
JAGP approach. Using a pseudopotential for the inert core of the Oxygen, with a
full optimization of the variational parameters, we obtain at the VMC level a
binding energy of -4.5(0.1) Kcal/mol, while LRDMC calculations gives -4.9(0.1)
Kcal/mol (experiment 5 Kcal/Mol). The calculated dispersion curve reproduces
both at the VMC and LRDMC level the miminum position and the curvature.The
quality of the WF gives us the possibility to dissect the binding energy in
different contributions by appropriately switching off determinantal and
Jastrow terms in the JAGP: we estimate the dynamical contribution to the
binding energy to be of the order of 1.4(0.2) Kcal/Mol whereas the covalent
contribution about 1.0(0.2) Kcal/Mol. JAGP reveales thus a promising WF for
describing systems where both dispersive and covalent forces play an important
role.
