Cluster-in-Cluster Approach for Computing MP2-Level Vibrational Infrared Spectra of Large Molecular Clusters

Constructing the Hessian matrix (HM) for large molecules demands huge computational resources.Here, we report a cluster-in-cluster (CIC) procedure for efficiently evaluating HM and dipole derivatives for large molecular clusters employing the secondorder Mller-Plesset perturbation (MP2) theory.The highlight of the proposal is to separate the estimations of Hartree-Fock (HF) and post-HF components.The parent cluster with n molecules is divided (virtually) into n sub-clusters centering each monomer and accommodating its near neighbors decided by a distance cutoff.The HF level HM is obtained by doing full calculation (FC), while the correlation is approximated by the respective sub-clusters.A software automating the procedure (followed by calculating infrared (IR) frequencies and intensities) is applied to deduce the IR spectrum for a variety of molecular clusters, particularly water clusters of various sizes, containing up to 2000 basis functions.The accuracy of the IR spectrum constructed using CIC is remarkable, with a substantial time advantage ( w.r.t.its FC counterpart).The reduced computational resources and tractability of the computations are other major benefits of the procedure.