Determining Proton Affinities using psi4

This is the 3rd post in a series outlining a workflow using freely available computational chemistry resources with python interfaces to evalute properties of gas-phase ions. A cursory search illustrates that there are a variety of computational packages with a direct python interface but interestingly, not all of these packages are current. PySCF appears to be a solid choice, however, some of the documentation/examples do not provide a direct means to calculate thermochemistry. GAMESS is another option but the python wrapper for this system has not been updated in almost a year and appears only compatible with select python 2.7 installations. After testing all of these options, it became clear that psi4 provided a tractable approach to optimize the geometry of molecules followed by a detailed thermochemical and frequency evaluation. The ipynb notebook illustrates the mechanism to not only optimize the geometry of water, but also determine the proton affinity. This latter property remains essential for describing the ionization behavior of target molecules along with a host of other chemical properties. In many literature reports a more detailed treatment of the energy terms is often presented, however, as a first pass this workflow yields a result that is in good aggreement with the literature value for water.