Charge transfer term

The charge transfer term arises from the transfer of charge (i.e. electrons) from occupied molecular orbitals on one molecule to unoccupied orbitals on the other molecule. This contribution is calculated as the difference between the energy of the supermolecule XY when this charge transfer is specifically allowed to occur, and an analogous calculation in which it is not. 

Figure 1 3. Contour plot of the electron density of CO, showing the magnitudes and directions of atomic and charge transfer dipoles (arrow length is proportional to magnitude). Arrow heads point to the negative end. The molecular dipole moment is given by the vector sum of charge transfer terms (p.c.t.) and the atomic polarizations ( ra p). Values were obtained at the DFT level using the B3LYP functional and the 6-31 1+G(3df) basis set. The SCF molecular dipole = 0.096 D the computed molecular dipole ( Jtc.t.[0] + Aa.p.[0] + Hc.JC] + Aa.p.[C]) = 0.038 au = 0.096 D, close to the experimental value of 0.1 10 D (15).

CT = charge-transfer terms (excluding the one-body charge-transfer components)... 

As to dispersion, all ASP-W models use terms dependent on the distance between center of mass, of the form R ", with n=6,7,8,9,10. In the case of ASP-W, an empirical site-site dispersion function has also been proposed. On the other hand, ASP-W2 and ASP-W4 include charge transfer terms described with exponential functions for each 0-H pair. 

The major differences in the two methods used are that the empirical method does not contain terms with explicit electron overlap dependence such as a charge transfer term but does include total intermolecular geometry optimization. 

Since the involved configuration function corresponds to the ligand-to-metal charge transfer (LMCT), its contribution is called the charge transfer term. 

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