N-body:Many-body QM:QM vibrational frequencies: Application to small hydrogen-bonded clusters

We present an efficient method for reproducing CCSD(T) (i.e., the coupled-cluster method with single, double and perturbative connected triple excitations) optimized geometries and harmonic vibrational frequencies for molecular clusters with the N-body:Many-body QM:QM technique. In this work, all 1-body through N-body interactions are obtained from CCSD(T) computations, and the higher-order interactions are captured at the MP2 level. The linear expressions from the many-body expansion facilitate a straightforward evaluation of geometrical derivative properties (e.g., gradients and Hessians). For (H2O)n clusters (n = 3–7), optimized structures obtained with the 2-body:Many-body CCSD(T):MP2 method are virtually identical to CCSD(T) optimized geometries. Harmonic vibrational frequencies calculated with this 2-body:Many-body approach differ from CCSD(T) frequencies by at most a few cm−1. These deviations can be systematically reduced by including more terms from the many-body expansion at the CCSD(T) level. M...