  We present a natural orbital-based implementation of the intermediate
Hamiltonian Fock space coupled-cluster method for (1,1) sector of Fock space.
The use of natural orbital significantly reduces the computational cost and can
automatically choose an appropriate active space. The new method retains the
charge transfer separability of the original intermediate Hamiltonian Fock
space coupled-cluster method and gives excellent performance for valence,
Rydberg, and charge-transfer excited states. It offers significant
computational advantages over the popular equation of motion coupled cluster
method for excitation energy calculations, especially when one is interested in
the calculation of large no excited states.
