2-body:Many-body QM:QM study of structures, energetics, and vibrational frequencies for microhydrated halide ions

Abstract

ABSTRACTThe optimised structures, electronic dissociation energies and harmonic vibrational frequencies of microhydrated halide ions have been probed with the MP2 and CCSD(T) quantum mechanical (QM) electronic structure techniques as well as with the 2-body:Many-body QM:QM method (2b:Mb CCSD(T):MP2) in which the 1- and 2-body interactions are computed at the CCSD(T) level of theory while all higher-order contributions are captured at the MP2 level of theory. In general, the accuracy and efficiency of the 2b:Mb approach previously demonstrated for neutral, homogeneous HF and HO clusters extends to these negatively charged heterogeneous systems. Results indicate the MP2 method struggles to provide a reliable description of the vibrational frequencies for hydrated fluoride clusters even though it performs well for the other hydrated halides (chloride, bromide and iodide). Consequently, the accuracy of the 2b:Mb CCSD(T):MP2 approach for fluoride clusters is poorer than the other halides, and MP2 is not a recommended low-level method for high-accuracy frequency analyses of hydrated fluoride complexes with the 2b:Mb method or related QM:QM approaches.