Segmented all-electron contracted sextuple zeta valence plus polarization function (6ZP) basis sets for the elements from H to Ar were constructed to be used in conjunction with the non-relativistic and Douglas-Kroll-Hess Hamiltonians. The scalar relativistic effect at the coupled cluster level of theory on atomization energy for a sample of molecules was discussed. Additional improvements in the atomization energies were achieved by applying corrections due to core/valence correlation, atomic spin-orbit effects, and addition of tight d and f functions on second-row elements. This leads to estimates for the heat of formation of gaseous boron and carbon tetrafluoride. (C) 2011 Elsevier B.V. All rights reserved.