Lowest order perturbative approximation to vibrational coupled cluster method in bosonic representation.

We propose a perturbative approximation to the vibrational coupled cluster method in bosonic representation to reduce the cost of calculating the cluster matrix elements by considering only the first order of S and σ for the construction of the effective Hamiltonian e σ e - S H e S e - σ . With the systematic analysis of the results of two molecules, H 2 O and 1,1-difluoroethylene, we find that the accuracy of the transition energies with such low order approximation is comparable to the fully converged VCCM. The lowest order perturbative approximation is used to calculate the cluster matrix elements of the vibrational coupled cluster method in bosonic representation. The computational scaling for cluster matric elements is reduced from iterative N6 to non-iterative N4. [ABSTRACT FROM AUTHOR]