Specialized modification to the G4 composite method for computing formation enthalpies of nitrophenyl and nitrogen-heterocycle organic compounds

Abstract

We propose modifications to the G4 composite method, which allows us to continue using the method as a black box for computing gas-phase formation enthalpies of closed-shell organic molecules, but with the following additional features. (a) The resultant predictions are drastically improved when the molecules contain the nitrophenyl and nitrogen-heterocycle moieties, relative to the original G4 recipe. (b) The proposed modified G4 recipes retain a similar predictive capability for computing formation enthalpies of small closed-shell organic molecules, which were part of the original training set of the G4 recipe. The modifications consist of re-parameterizing two empirical parameters related to the high-level correction term and the frequency scaling factor. The latter is related to both the zero point energy correction as well as to some anharmonic effects that impact the vibrational partition function. We also show that the method used to optimize geometries and to calculate frequencies does not significantly impact the quality of the predictions (after the respective reparameterization). Here, the functionals ωB97XD and M06-2X as well as the MP2 method were tested. In contrast, we show that the chemical information contained in the set used to determine the empirical parameters plays a major role in the overall agreement with experimental formation enthalpies. Molecules that are not closed-shell organic are not studied.