  Explicitly correlated calculations of interaction energies with wave
functions that include all interparticle distances have suffered so far from
the lack of size-consistency resulting from the difficulty to define monomer
energies corresponding to the applied dimer basis. As a consequence it has not
been possible to obtain interaction energies vanishing at infinite intermonomer
distance $R$. This has dramatically reduced the accuracy of calculations at
distances where the error in the dimer energy was comparable with the
interaction energy itself. The same problem occurs in calculations of
interaction-induced properties. In this communication we show how to circumvent
this difficulty and obtain interaction energies or interaction-induced
properties that vanish at large $R$. This is achieved by relaxing the Pauli
principle in the diagonalization of the Hamiltonian of noninteracting monomers.
The basis functions used for this diagonalization belong to the representation
of the permutation group of the dimer induced by the product of representations
appropriate for the monomer spin states. Nonlinear parameters of the basis set
are optimized only for the dimer in the Pauli-allowed sector of the Hilbert
space. In this way, one obtains $R$-dependent energy of noninteracting monomers
and the corresponding interaction energy includes a counterpoise correction for
the basis set superposition error. The efficiency of this procedure is
demonstrated for the interaction of two hydrogen atoms where accurate reference
data are known.
