Highly accurate calculations of molecular electronic structure

The highly accurate calculation of molecular electronic structure requires the expansion of the molecular electronic wavefunction to be as nearly comp lete as possible both in one- and n-electron space. In this review, we cons ider the convergence behaviour of computed electronic energies, in particul ar electronic enthalpies of reaction, as a function of the one-electron spa ce. Based on the convergence behaviour, extrapolations to the limit of a co mplete one-electron basis are possible and such extrapolations are compared with the direct computation of electronic energies near the basis-set limi t by means of explicitly correlated methods. The most elaborate and accurat e computations are put into perspective with respect to standard and-from a computational point of view-inexpensive density functional, complete basis set (CBS) and Gaussian-2 calculations. Using the explicitly correlated cou pled-cluster method including singles, doubles and non-iterative triples re placements, it is possible to compute (the electronic part of) enthalpies o f reaction accurate to within 1 kJ mol(-1). To achieve this level of accura cy with standard coupled-cluster methods, large basis sets or extrapolation s to the basis-set limit are necessary to exploit fully the intrinsic accur acy of the coupled-cluster methods.