We have improved our multi-level electronic structure methods MLSEn for calculating the atomization energies and reaction energy barriers for neutral systems by using improved correlation-consistent basis sets for second-row elements. The re-parameterization of the improved methods MLSEn + d was based on updated databases of 109 atomization energies, 38 hydrogen-transfer barrier heights, and 22 neutral reaction barrier heights from a recently developed database of non-hydrogen-transfer reactions. The improved methods perform very well on all three types of energies with mean unsigned errors of 0.70, 0.87, and 0.69 kcal/ mol by the MLSE4 + d method. (c) 2005 Elsevier B.V. All rights reserved.