Localized Orbitals in the Theory of Correlated Wavefunctions

Abstract

The proposition that localized orbitals are the orbitals most suitable for extending the wavefunction to take into account electron correlation is examined in the light of recent calculations of correlation energies. It is found, in agreement with the results of other authors, that the total interorbital correlation energy may be expected to be larger than the intraorbital energy in many systems of chemical interest, and that the ratio of these contributions is probably rather insensitive to a transformation of orbitals. It is also found that the correlation between electrons with like spins can make a considerable contribution to the correlation energy, and that this contribution is invariant with respect to a transformation of orbitals. The concept of nearest‐neighbor orbitals is used to propose a new localization criterion which minimizes the interaction between electrons in orbitals localized on different atoms in a molecule. The dependence on the orbitals of the two‐particle density function is examined. Orbital correlation factors are introduced which provide information as to the dependence of electron correlation on the distribution of electrons in the orbital approximation.