Electron correlation in chemical bonds

We study the correlation energy in the H-2 molecule as a function of bond distance and the electron coupling parameter that modifies the strength of electron-electron repulsion. For the unscaled electron repulsion we find an unusual complexity of the correlation energy dependence on the bond distance. This complexity remains if the electron repulsion is scaled down. We pay particular attention to the range of bond distances near the H-2 equilibrium geometry. We confirm that good performance of the modern exchange-correlation functionals in molecular geometry optimization is due to the cancellation of errors between the exchange and correlation components. These components have different functional dependence on the electron coupling parameter, so the cancellation of errors no longer occurs when the electron repulsion is scaled down. This observation is important if the adiabatic connection theorem is used to construct the exchange-correlation functionals. It is also relevant for the performance of exchange-correlation functionals to study ionic interactions. The performance of the correlation operator in the molecular environment is also investigated and discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)30109-X].