Present status of the correlation problem

Nowadays about 90% of the quantum-mechanical calculations on molecules are performed by the self-consistent-field (SCF) method, using more or less extended sets of basis functions without any consideration of the possible effects of correlation. There are, no doubt, two main reasons for this trend: (1) the ready availability of efficient SCF programmes, and (ii) the totally unexpected success of the resultant Hartree-Fock (or near-Hartree-Fock) wavefunctions as a means of predicting molecular properties at a quantitative level. Nevertheless, even if it might now be considered a minority interest, important work on electron correlation is in progress and until it has reached fruition, probably within the next decade, 'chemical accuracy' for systems of moderate size will not be achieved. This paper attempts a reappraisal and an exposition of current trends. (Auth.)