Comparing the accuracy of perturbative and variational calculations for predicting fundamental vibrational frequencies of dihalomethanes (2018)
Type of ContentJournal Article
- Science: Journal Articles 
Alternative TitleHow accurately can perturbative and variational vibrational models predict the fundamental frequencies of dihalomethanes?
Three dihalogenated methane derivatives (CH2F2, CH2FCl, and CH2Cl2) were used as model systems to compare and assess the accuracy of two different approaches for predicting observed fundamental frequencies: canonical operator Van Vleck vibrational perturbation theory (CVPT) and vibrational configuration interaction (VCI). For convenience and consistency, both methods employ the Watson Hamiltonian in rectilinear normal coordinates, expanding the potential energy surface (PES) as a Taylor series about equilibrium and constructing the wavefunction from a harmonic oscillator product basis. At the highest levels of theory considered here, fourth-order CVPT and VCI in a harmonic oscillator basis with up to 10 quanta of vibrational excitation in conjunction with a 4-mode representation sextic force field (SFF-4MR) computed at MP2/cc-pVTZ with replacement CCSD(T)/aug-cc-pVQZ harmonic force constants, the agreement between computed fundamentals is closer to 0.3 cm-1 on average, with a maximum difference of 1.7 cm-1. The major remaining accuracy-limiting factors are the accuracy of the underlying electronic structure model, followed by the incompleteness of the PES expansion. Nonetheless, computed and experimental fundamentals agree to within 5 cm-1, with an average difference of 2 cm-1, confirming the utility and accuracy of both theoretical models. One exception to this rule is the formally IR-inactive but weakly allowed through Coriolis-coupling H-C-H out-of-plane twisting mode of dichloromethane, whose spectrum we therefore revisit and reassign. We also investigate convergence with respect to order of CVPT, VCI excitation level, and order of PES expansion, concluding that premature truncation substantially decreases accuracy, although VCI(6)/SFF-4MR results are still of acceptable accuracy, and some error cancellation is observed with CVPT2 using a quartic force field.
CitationKrasnoshchekov SV, Schutski RS, Craig NC, Sibaev M, Crittenden DL (2018). Comparing the accuracy of perturbative and variational calculations for predicting fundamental vibrational frequencies of dihalomethanes. Journal of Chemical Physics. 148(8). 084102-.
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KeywordsCanonical Van Vleck Perturbation Theory; Vibrational Configuration Interaction; Sextic Force Field; Halogenated Methanes
ANZSRC Fields of Research34 - Chemical sciences::3405 - Organic chemistry::340505 - Physical organic chemistry
34 - Chemical sciences::3407 - Theoretical and computational chemistry::340701 - Computational chemistry
34 - Chemical sciences::3407 - Theoretical and computational chemistry::340704 - Theoretical quantum chemistry
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