Theory of Correlated Wavefunctions. III. Alternative Initial Approximations

摘要

The perturbation treatment of electron correlation is generalized to include an initial orbital approximation PSgr;0in which the orbitals are arbitrary within a unitary transformation which leaves PSgr;0unchanged. Various methods are considered for the partitioning of the total Hamiltonian into zerothhyphen; and firsthyphen;order parts consistent with a given zerothhyphen;order PSgr;0. One particular partitioning is discussed in detail which leads to simple and physically significant expressions for the secondhyphen; and thirdhyphen;order energies, and for which the secondhyphen;order energy gives a good estimate of the correlation energy when the orbitals are well localized and when no nearhyphen;degeneracies are present. A calculation of the angular correlation in the ground state of the beryllium atom serves to investigate the validity of a number of concepts.