Communication: Active-space decomposition for molecular dimers

We have developed an active-space decomposition strategy for molecular dimers that allows for the efficient computation of the dimer's complete-active-space wavefunction while only constructing the monomers' active-space wavefunctions. Dimer states are formed from linear combinations of direct products of localized orthogonal monomer states and Hamiltonian matrix elements are computed directly without explicitly constructing the product space. This decomposition is potentially exact in the limit where a full set of monomer states is included. The adiabatic states are then found by diagonalizing the dimer Hamiltonian matrix. We demonstrate the convergence of our method to a complete-active-space calculation of the full dimer with two test cases: the benzene and naphthalene dimers.