The VCI-P code: An iterative variation-perturbation scheme for efficient computations of anharmonic vibrational levels and IR intensities of polyatomic molecules
Résumé
A variation-perturbation scheme called VCI-P code is proposed. It is developed mainly for the efficient and accurate theoretical description of vibrational resonances in polyatomic molecules. This state-specific process consists of the iterative construction of small matrices for each vibrational state, using the most important configurations contributing to that state. The weak couplings are considered perturbationally. The keypoint of this paper is a recipe that allows the massive truncation of the vibrational configuration space with minimum error in the calculated energies. The anharmonic frequencies and IR/vibrational absorption (VA) intensities obtained using VCI-P for methane and formaldehyde are compared to their full VCI counterparts. The convergence of the VCI-P results with respect to configuration selection is also discussed from the examples of trans-difluoroethylene and ethylene oxide (also called oxirane). A parallelization scheme for the 3N - 5 calculations on distributed memory computers is proposed. Representative computational times are presented for molecules ranging in size from 4 to 15 atoms. © Springer-Verlag 2009.