  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.
