Charge-density concentration and electron-electron coalescence density in atoms and molecules

Values for the average electron density [rho] and the electron-electron coalescence density or intracule density at the origin I(0) have been calculated for a number of atoms and molecules using several ab initio methodologies. [rho] can be taken as a measure of local one-electron density concentration, and shows little dependence on the theoretical level employed. In contrast, I(0) has found to be highly dependent on the level of calculation. In particular, for a given nb initio method, the difference [rho]/4-I(0) can be related to the degree of Coulomb electron correlation introduced by that method in an atomic or molecular system.