A New Split Charge Equilibration Model and Repeat Electrostatic Potential Fitted Charges for Periodic Frameworks with A Net Charge

Periodic frameworks that possess a net charge, such as zeolites, are an important class of materials in wide use. For guest-host interactions to be simulated in these materials, partial atomic charges are often used. In this work, we investigate two methods for the generation of partial atomic charges in periodic systems having a net framework charge. We first examine the validity of generating REPEAT electrostatic potential fitted charges derived from periodic electronic structure calculations, where a constant background charge is added to neutralize the net charge on the framework. The constant background charge obviates the need to add neutralizing counterions, which may induce artifacts such as polarization in the infinite periodic system. The second method we explore is the split charge equilibration (SQE) method for the rapid generation of partial atomic charges. The original formulation of the SQE method cannot be applied to systems with a net charge. In this work, we reformulate the SQE method by transforming the split charges into an atomic charge basis that allows for non-neutral systems to be treated. The new SQE model, which we call SQE(AB) (for atomic basis), was validated with a series of tests using both charged and neutral metal organic frameworks and zeolites. It was shown that SQEAB gives equivalent results to those of the original SQE model for neutral systems. We then demonstrated that the SQE(AB) method is able to "capture" the chemical structure of a charged framework better than that of the charge equilibration model by comparing to REPEAT electrostatic potential fitted charges.