Quantum mechanical studies of HeH+ and LiH.

Wavefunctions possessing effective nuclear charges which are explicitly angularly dependent have been used as a simple minimal basis set for a one-centre study of HeH+. Correlation effects were introduced by using spatially different orbitals for different electrons of opposite spins. Allowance for the effects of correlation and also molecular formation within the confines of the model are discussed in terms of changes which occur in the electron density distribution. A comparison is made with results obtained from a corresponding analysis of an extensive, single-centre, configuration-interaction wavefunction. Although several points of similarity emerge, the analysis primarily indicates features and shortcomings of the basis orbitals which should aid any future applications. Also a study has been made of the LiH molecule which utilizes the one-particle density for an analysis of the effects of electron correlation. Two wavefunctions are examined in detail for their effectivness in representing correlation. The first of these is the GI method which uses spin projection to give a wavefunction which may be interpreted by means of the Independent Particle Model, the second is a very accurate configuration-interaction calculation. Correlation effects are observed within the model by constructing the uncorrelated wavefunction from the appropriate doubly occupied first natural configuration calculated for each of the above cases. The analysis is presented in terms of density difference maps and profiles which render the results in a particularly clear and revealing form.