Charge-transfer energy through the dipole moment

The charge-transfer energy contribution is one of the most controversial components of the total interaction energy. Commonly, the energy associate to a charge-transfer process depends on population analysis. Therefore, the results further depend on how the population analysis is defined, and certainly, the results may be arbitrary. Moreover, another important feature of the current methods is the basis sets dependency. The results of methodologies that depend on orbital-based population analyses tend to have a strong dependency on the size of the basis set utilized. This basis set dependency is eliminated by using spatial partitioning population analyses. However, these methodologies still rely on the arbitrary choice of how to divide the space. In this work, we study the use of the molecular dipole moment as a reference to describe the charge transfer-free system, i.e., a system in which the charge-transfer process is avoided. We use the recently developed constrained dipole moment density functional theory methodology to constrain the dipole moment of several systems according to reference values. These dipole moment references do not present charge transfer nor polarization contributions. In this manner, we have calculated the charge-transfer energy contributions and the total interaction energies of 13 non-covalent complexes. In addition, we determined two long range charge-transfer excitations considering the dipole moment as a reference. The calculated charge-transfer energy contributions and excitation energies are in a very good agreement with the fragment-based Hirshfeld methodology. Nevertheless, the constrained dipole moments results do not depend on population analysis. Moreover, the method is robust with respect to the strength of the charge transfer and the basis set size.