The estimation of net atomic charges from calculated electron densities

9 The estimation of net atomic charges from calculated electron densities 

The constant electron density contour diagrams of one homonuclear and five heteronuclear diatomic molecules presented in Fig. 8.6 have been obtained by reasonably accurate quantum chemical calculations. The electric dipole moment of F2 is zero by symmetry. The electric dipole moments of heteronuclear molecules like LiFl, LiF, HF, CIF or CO may be calculated from their electron densities using equation (5.2). These dipole moments, in turn, allow us to calculate the ionic characters gic(calc) = /Xei(calc)(calc). In Table 8.1 we compare the calculated ionic characters of 21 heteronuclear diatomic molecules with their experimental counterparts. The agreement between experiment and calculations is good the average deviation between experimental and calculated values is less than 0.02 a.u, the maximum deviation (in KLi) is 0.05 a.u. 

The so-called Atoms In Molecules (AIM) approach developed by Bader and coworkers [3] is radically different. Topological analysis of the electron density is used to draw boundary surfaces between the atoms, and the negative charge on each atom is found by integration of the total electron density within its boundaries. Addition of the nuclear charge gives the net charge, aim. 

We now consider a heteronuclear diatomic molecule as, for instance, LiH. The first step in the topological analysis is to find the saddle point (the BCP) on the electron density 

It should be recalled that the validity of the simple expression for the dipole moment of a diatomic molecule AB 

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