%0 DATA
%A Mark, Iron
%A Jonathan, Gropp
%D 2019
%T Using Cost-Effective Density Functional Theory (DFT) to Calculate Equilibrium Isotopic Fractionation for Reactions Involving Large Organic Molecules
%U https://chemrxiv.org/articles/Using_Cost-Effective_Density_Functional_Theory_DFT_to_Calculate_Equilibrium_Isotopic_Fractionation_for_Reactions_Involving_Large_Organic_Molecules/8063654
%R 10.26434/chemrxiv.8063654.v1
%2 https://chemrxiv.org/ndownloader/files/15026504
%2 https://chemrxiv.org/ndownloader/files/15026498
%K density functional theory
%K isotopic fractionation
%K Bigeleisen-Mayer equation
%K density fitting
%K vibrational frequency
%X With an eye towards applications in environmental sciences, the factors impacting the accuracy of calculating equilibrium isotopic fractionation α are considered. α is calculated from the reduced partition function relation β via the vibrational frequencies. Density-fitting (DF, or resolution of the identity – RI) is a popular tool to help reduce the cost of DFT energy calculations, and its impact on the accuracy of calculating vibration frequencies was determined to be negligible provided one uses a sufficiently large auxiliary density-fitting basis set. Using a set of high-accuracy fractionations for a series of small molecules, the accuracy of several density-functional theory (DFT) exchange–correlation functionals in calculating β was considered. Based on these results, a selection of functionals was tested against two benchmark databases of isotopic fractionations that were generated, one for D/H fractionation (HEIF11) and one for heavy-atom fractionation (CNOEIF35). It was found that, with the def2-TZVP basis set, O3LYP had the lowest mean absolute deviation (21‰ and 3.9‰, respectively), but the GGA/meta-GGA functionals τHCTH_{D3BJ}, τHCTH and HCTH have almost similar performances (22‰ and 4.1‰, respectively, for τHCTH_{D3BJ}). In this work we provide a roadmap to predict for equilibrium isotopic fractionations of large organic molecules, and constrain their potential uncertainties.