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A theory for calculating vibrational energy levels and infrared intensities is developed in the equation of motion framework at the random phase approximation level. The vibrational Hamiltonian is expanded in the harmonic oscillator ladder operators making a Hamiltonian a bosonic Hamiltonian. The excitation operator is expanded to include at most two creations and two annihilation operators making it equivalent to the random phase approximation. The method is applied for the calculation of vibrational spectral properties of two molecules. The results are found to be satisfactory, making this approach a viable option for large molecular systems. A theory for calculating vibrational energy levels and infrared intensities is developed in the equation of motion framework at the random phase approximation level. The vibrational Hamiltonian is expressed in bosonic representation. The excitation operator is expanded to include at most two creation and annihilation operators. [ABSTRACT FROM AUTHOR]

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]