An accurate and detailed semiempirical intermolecular potential energy surface for (HCl)(2) has been determined by a direct nonlinear least-squares fit to 33 microwave, far-infrared and near-infrared spectroscopic quantities using the analytical potential model of Bunker et al. [J. Mel. Spectrosc. 146, 200 (1991)] and a rigorous four-dimensional dynamical method (described in the accompanying paper). The global minimum (D-e=-692 cm(-1)) is located near the hydrogen-bonded L-shaped geometry (R=3.746 Angstrom, theta(1)=9 degrees, theta(2)=89.8 degrees, and phi=180 degrees). The marked influence of anisotropic repulsive forces is evidenced in the radial dependence of the donor-acceptor interchange tunneling pathway. The minimum energy pathway in this low barrier (48 cm(-1)) process involves a contraction of 0.1 Angstrom in the center of mass distance (R) at the C-2h symmetry barrier position. The new surface is much more accurate than either the ab initio formulation of Bunker et al. or a previous semiempirical surface [J. Chem. Phys. 78, 6841 (1983)].