Analysis of charge transfer effects in molecular complexes based on absolutely localized molecular orbitals

Abstract

A new method based on absolutely localized molecular orbitals (ALMOs) is proposed to measure the degree of intermolecular electron density delocalization (charge transfer) in molecular complexes. ALMO charge transfer analysis (CTA) enables separation of the forward and backward charge transfer components for each pair of molecules in the system. The key feature of ALMO CTA is that all charge transfer terms have corresponding well defined energetic effects that measure the contribution of the given term to the overall energetic stabilization of the system. To simplify analysis of charge transfer effects, the concept of chemically significant complementary occupied-virtual orbital pairs (COVPs) is introduced. COVPs provide a simple description of intermolecular electron transfer effects in terms of just a few localized orbitals. ALMO CTA is applied to understand fundamental aspects of donor-acceptor interactions in borane adducts, synergic bonding in classical and nonclassical metal carbonyls, and multiple intermolecular hydrogen bonds in a complex of isocyanuric acid and melamine. These examples show that the ALMO CTA results are generally consistent with the existing conceptual description of intermolecular bonding. The results also show that charge transfer and the energy lowering due to charge transfer are not proportional to each other, and some interesting differences emerge which are discussed. Additionally, according to ALMO CTA, the amount of electron density transferred between molecules is significantly smaller than charge transfer estimated from various population analysis methods.

Microscopic properties of liquid water from combined ab initio molecular dynamics and energy decomposition studies

“…In this review, we discuss a novel energy decomposition analysis scheme based on absolutely localized molecular orbitals (ALMO EDA) 45,46 . Unlike conventional MOs, which are generally delocalized over all molecules in the system, ALMOs are expanded in terms of the atomic orbitals of only a given molecule [47][48][49][50] .…”

“…Unlike conventional MOs, which are generally delocalized over all molecules in the system, ALMOs are expanded in terms of the atomic orbitals of only a given molecule [47][48][49][50] . Although ALMOs were originally used to speed up the calculation of SCF energies for large ensembles of molecules 50 , they are now widely used in energy decomposition analysis (EDA) [44][45][46] . It should be mentioned that since the introduction of ALMO EDA 45 , ALMO-based decomposition methods have been extended to many-determinant wave functions 51 .…”

“…Charge transfer analysis. In addition to the energy decomposition, ALMOs have been used to measure the amount of electron density transferred between molecules 46 . The intermolecular CT in such a charge transfer analysis (ALMO CTA), ∆Q DEL , is defined as the change in the electron density from the polarized R P OL state to the fully converged state R SCF .…”

“…This freedom can be used to find new sets of orbitals for x and y, in which charge transfer from x to y is described as each occupied orbital on x donating electrons to only one (complementary) virtual orbital on y. Such orbitals are called complementary occupied-virtual pairs (COVPs) 46 .…”

Charge Transfer from Regularized Symmetry-Adapted Perturbation Theory

“…While these components are not usually separated in a SAPT calculation of the interaction energy, there are approximate methods, such as that introduced by Stone [1] and used by Stone & Misquitta [2] in the context of a variant of SAPT based on a density-functional description of the monomers (termed SAPT(DFT)) [2], which enable us to make a reasonable partitioning of the induction energy-at least at second-order-into its charge-transfer and polarization constituents. Though these methods, and their supermolecular counterparts [3], provide sensible charge-transfer energies, particularly for complexes at their equilibrium configurations, they exhibit serious drawbacks, all related to their basis-set dependence for small intermolecular separations. Furthermore, as we shall demonstrate in this paper, the Stone & Misquitta procedure leads to charge-transfer energies that do not seem to reflect the physical nature of the process.…”