We present a new reduced scaling MRSDCI algorithm based upon the combination
of local correlation methods and pseudospectral methods. The weak-pairs
approximation of Saebo and Pulay is employed to eliminate configurations
having excitations out of pairs of weakly-interacting orbitals. In
conjuction with this, the pseudospectral approximation is used to reduce the
certain two electron integrals into a product of intermediate quantities
depending on no more than two orbital indices. The intermediate quantities
are used directly in the CI equations to substantially reduce the number of
operations required for Hamiltonian diagonalization. Additionally, our CI
algorithm is based upon the symmetric group graphical approach CI of Duch
and Karwowski. For the purpose of developing reduced scaling CI algorithms,
this approach has important advantages. The most important of these
advantages are the on-the-fly calculation of coupling coefficients and the
separation of the spin and spatial parts of the wavefunction. The MRSDCI
method presented here is up to 5 fold faster than the analytic non-local
method. We apply the method to determine a series of binding energies in
hydrocarbons and show that the method predicts binding energies which are
within a few kcal/mol of those predicted by the analytic non-local method.
Copyright. All Rights Reserved.