Incremental vibrational configuration interaction theory, iVCI: Implementation and benchmark calculations.

The implementation of an algorithm for the determination of vibrational state energies based on a many-body expansion within the framework of configuration interaction theory is presented. An efficient evaluation of the increments within this approach is realized by an iterative configuration selection scheme. The new algorithm is characterized by low memory demands and an embarassingly parallel workload. The convergence of the expansion has been studied for a series of small molecules of increasing size, namely, formaldehyde, ketene, ethylene, and diborane. A threshold function has been employed to reduce the number of increments for high orders of the expansion. Benchmark calculations with respect to customary configuration-selective vibrational configuration interaction calculations are provided.