A priori calculation of molecular properties to chemical accuracy

Quantum chemical methods for the calculation of molecular properties to chemical accuracy are reviewed. We begin by reviewing wave-function based electronic structure theory, with emphasis on coupled-cluster theory and the description of electron correlation in terms of virtual excitations from occupied to virtual molecular orbitals. Next, we discuss the expansion of molecular orbitals in atomic orbitals and the design of one-electron basis sets for correlated calculations at the coupled-cluster level of theory. In particular, we discuss the convergence of the electronic energy in the principal expansion as realized in the correlation-consistent basis sets. Following this theoretical introduction, we consider the accurate calculation of atomization energies, reaction enthalpies, dipole moments and spectroscopic constants such as bond distances and harmonic and anharmonic force constants. For each property, we identify what levels of theory in terms of basis set (the one-electron description) and virtual excitation level (the N-electron description) are needed for agreement with experiment to chemical accuracy, that is, about 1 kcal mol(-1) (1 kcal = 4.184 kJ) for atomization energies and reaction enthalpies, about 0.1 pm for bond distances and about 1 cm(-1) for vibrational frequencies. In each case, we consider in detail the direction and magnitude of the changes in the calculated properties with improvements in the one- and N-electron descriptions of the electronic system, paying particular attention to the cancellation of errors arising from the simultaneous truncations of the coupled-cluster expansion and the one-electron basis set. We emphasize that agreement with experiment, even with chemical accuracy, for a few selected properties is by itself no guarantee of quality and should never be taken as indicative of an accurate description of the electronic system. To ensure such a description, the errors arising from the truncations of the one- and N-electron expansions must be controlled by carrying out sequences of calculations, where the different levels of theory are systematically varied and where convergence is carefully monitored. Copyright 2004 John Wiley Sons, Ltd.