This work systematically examines the basis set convergence of MP2 and CCSD(T) harmonic vibrational frequencies of small water and hydrogen fluoride clusters with basis sets as large as aug-cc-pV6Z.  The performance of the augmented correlation consistent triple-zeta quality basis sets is not significantly better than the corresponding double-zeta ones.  In general, basis sets of quadruple-zeta quality are required to get reasonably close to the estimated complete basis set (CBS) limit frequencies, but this convergence can be accelerated with explicitly correlated (F12 or R12) methods.  Comparison of MP2 and CCSD(T) data reveals sizable discrepancies not only in the computed vibrational frequencies but also in the frequency shifts induced by hydrogen bonding.  These observations suggest that reliable frequency computations should employ the CCSD(T) method and quadruple-zeta basis sets (or possibly the CCSD(T)-F12 method with triple-zeta basis sets).   The analytic Hessians recently developed by our research group [1] for the N-body:Many-body integrated QM:QM method provide a means to overcome the unfavorable scaling of canonical CCSD(T) computations and extend benchmark harmonic vibrational frequency calculations to larger molecular clusters held together by hydrogen bonds and other non-covalent interactions. All 1-body through N-body contributions are computed with a high-level method, while higher-order interactions are evaluated with a less demanding low-level method.  For H2O and HF clusters, the CCSD(T):MP2 combination of the 2-body:Many-body approach provides harmonic vibrational frequencies within a few wavenumbers of the canonical CCSD(T) frequencies while substantially reducing the computational time.  If necessary, these deviations can be systematically reduced to less than 1 wavenumber through the corresponding 3-body:Many-body and 4-body:Many-body procedures.  This N-body:Many-body integrated QM:QM procedure has also been extended to the evaluation anharmonic vibrational frequencies and of infrared (IR) intensities via second-order vibrational perturbation theory (VPT2). [1] Howard, J.C.; Tschumper, G.S.; N-body:Many-body QM:QM vibrational frequencies: Application to small hydrogen-bonded clusters, J. Chem. Phys., 2013 , 139, 184113.  DOI: 10.1063/1.4829463

Pursuing the CCSD(T) complete basis set limit for the vibrational frequencies of water and other hydrogen bonded clusters:  The future of computational chemistry is so bright you've got to wear shades

248th National Meeting (2014)

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