Ab Initio Effective One-Electron Potential Operators. II. Applications for Exchange-Repulsion Energy in Effective Fragment Potentials

The concept of the effective one-electron potentials (OEP) has been useful for many decades in efficient description of electronic structure of chemical systems, especially extended molecular aggregates such as interacting molecules in condensed phases. Here, a general method for effective OEP-based elimination of electron repulsion integrals (ERI), that is tuned towards the fragment-based calculation methodologies such as the second generation of the effective fragment potentials (EFP2) method, is presented. Two general types of the OEP operator matrix elements are distinguished and treated either via the distributed multipole expansion or the extended density fitting schemes developed in this work. The OEP technique is then applied to address the problem of using incomplete EFP2 settings in many applications in interaction energy and molecular dynamics simulations due to relatively high computational cost of evaluating the charge transfer (CT) effects as compared to other effects. The alternative OEP-based CT energy model is proposed in the context of the intermolecular perturbation theory with Hartree-Fock non-interacting gas-phase reference wavefunctions, compatible with the EFP2 formulation. It is found that the computational cost can be reduced up to 20 times as compared to the CT energy method within the EFP2 scheme without compromising the accuracy for a wide range of weakly interacting neutral molecular complexes. Therefore, it is believed that the proposed model can be used within the EFP2 framework, making the CT energy term no longer the bottleneck in EFP2-based simulations of complex systems.