The performance of explicitly correlated wavefunction for the computation of anharmonic vibrational frequencies

Author's abstract: Explicitly correlated coupled cluster singles, doubles, and perturbative triples level [CCSD(T)-F12] and second-order closed-shell Møllar-Plesset perturbative theory (MP2-F12) anharmonic vibrational frequencies are compared with gas phase experiment and higher-level computations. These involve CCSD(T) with the complete basis set limit extrapolation along with, an additive factor for the energy difference between core and noncore electrons and a similar additive difference for scalar relativity to give the CcCR approach. One hundred and sixty-nine vibrational frequencies are computed using CCSD(T)-F12 with the aug-cc-pVTZ basis set. For MP2-F12 twenty-five molecules are examined under aug-cc-pVDZ basis set. The CCSD(T) computational analysis of the closed-shell molecules' anharmonic vibrational frequencies resulted in a mean absolute error of 7.5 cm-1 between the CCSD(T)-F12 and CcCR. Comparison of the CCSD(T)-F12 with gas phase experiment gave mean absolute error of 5.8 cm-1 for twenty-nine closed-shell vibrational frequencies. However, the total computational time is reduced by more than 4 orders of magnitude when utilizing CCSD(T)-F12 as opposed to CcCR. From the computation, it is clear CCSD(T)-F12 correlated wave function is superior to MP2-F12, that this approach is a viable means of computing anharmonic vibrational frequencies of molecules when CcCR is not feasible to use.