Chemical accuracy from 'Coulomb hole' extrapolated molecular quantum-mechanical calculations

In standard ab initio molecular quantum-mechanical calculations it is diffi cult, if at all possible, to calculate fundamental chemical properties like atomisation energies and reaction enthalpies to the chemical accuracy of 1 kcal/mol approximate to 4 M/mol. The principal obstacle is the presence of the Coulomb hole in the electronic wave function, whose description conver ges slowly with the size of the basis set. From a systematic sequence of ba sis-set calculations, we demonstrate that physically motivated extrapolatio ns can be used to accelerate the basis-set convergence significantly and th at, for highly correlated wave functions, chemical accuracy is obtained eve n with medium-sized basis sets when the extrapolation is used. (C) 2001 Els evier Science B.V. All rights reserved.