The computational cost of coupled cluster and EOM-CC method can be significantly reduced by excluding the higher virtual orbitals from the correlation calculations. Direct truncation of the higher energy canonical virtual orbitals can result in a large error because of the slow convergence of the dynamic correlation effects with the virtual orbital space . However, the natural orbital has been shown to give a fast convergence of correlation energy with respect to the size of the virtual orbitals. Natural orbitals are the eigenfunctions of the correlated one-particle reduced density matrix. Among the various flavors of natural orbital available, we have used the frozen natural orbital , where the occupied orbital is kept frozen to canonical Hartree-Fock orbitals.