We benchmark how accurate VPT2 and VSCF IR peak position estimation is relative to <i>broadened</i> experiment. That is, we consider the already broadened experimental data and estimate the observed, experimental peak center from the superposition of vibrational excitations. We consider 22 molecular systems in the gas and liquid phases, respectively. We find that VSCF/MBPT(2)’s accuracy is no better than VPT2/KS-DFT; given VSCF’s much greater cost, we thereafter focused only on VPT2. Using VPT2/B971/aug’-cc-pVDZ, we observe the accuracy of IR spectral peak position is 1.4 ± 4 % (μ ± 2σ) of the frequency being estimated. It is important to note that the absolute error increases as a function of the frequency. A histogram of the errors confirms that this error has a systematic bias. By contrast, if one uses MBPT(2)/aug’-cc-pVTZ, the distribution of errors reduces to 0.4 ± 2% (μ ± 2σ) of the frequency, showing a very small bias and much smaller random error. The liquid phase is approximated by a cluster model for the sake of IR spectral prediction. We observe that this is an excellent approximation to describe liquid IR spectra except for cases of organic acids. The liquid error, not counting organic acids, is the same as gaseous error.

IR peak position accuracy compared to experiment from VPT2 and VSCF anharmonic formalisms for gaseous and liquid systems

ACS Spring Meeting 2021

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