Assessment of DFT for Computing Sum Frequency Generation Spectra of an Epoxydiol and a Deuterated Isotopologue at Fused Silica/Vapor Interfaces

We assess the capabilities of eight popular density functional theory (DFT) functionals, in combination with several basis sets, as applied to calculations of vibrational sum frequency generation (SFG) spectra of the atmospherically relevant isoprene oxidation product trans-β-isoprene epoxydiol (IEPOX) and one of its deuterated isotopologues at the fused silica/vapor interface. We use sum of Squared Differences (SSD) and Total Absolute Error (TAE) calculation to estimate the performance of each functional/basis set combination in producing SFG spectra that match experimentally obtained spectra from trans-β-IEPOX and one of its isotopologues. Our joined SSD/TAE analysis shows that while the twist angle of the methyl C3v symmetry axis of trans-β-IEPOX relative to the surface is sensitive to the choice of DFT functional, the calculated tilt angle relative to the surface normal is largely independent of the functional and basis set. Moreover, we report that hybrid functionals such as B3LYP, ωB97X-D, PBE0 and B97-1 in combination with a modest basis sets, such as 6-311G(d,p), provide good agreement with experimental data and much better performance than pure functionals such as PBE and BP86. However, improving the quality of the basis set only improves agreement with experimental data for calculations based on pure functionals. A conformational analysis, based on comparisons of calculated and experimental SFG spectra, suggests that trans-β-IEPOX points all of its oxygen atoms towards the silica/vapor interface.