  We propose an algorithm, that scales with the fifth power of the system size,
for computing the second-order dispersion energy for monomers described with
multiconfigurational wave functions. This scaling can be achieved when the
number of virtual (unoccupied) orbitals largely exceeds the number of active
orbitals, which is the case in practical calculations. Our approach employs
Cholesky decomposition of Coulomb integrals and a recently developed recursive
formula for density response functions of the monomers, enabling dispersion
energy computations for systems in nondegenerate ground or excited states with
arbitrary spin. As a numerical illustration, we apply the new algorithm in the
framework of multiconfigurational symmetry adapted perturbation theory,
SAPT(MC), to study interactions in dimers with localized excitons. The SAPT(MC)
analysis reveals that the dispersion energy may be the main force stabilizing
excited-state dimers.
