A new and less expensive G2-type approach, G2MS, which can be used for accurate energy prediction for up to seven to eight atoms has been proposed and tested against the standard G2 data set. The results compare well with other G2 methods. The G2MS method performs an extrapolation of correlation and basis set effects, while the integrated MO+MO (IMOMO) method provides an extrapolation of electronic and steric effects from a small model to a large real system. Thus, using G2MS as the high-level method in IMOMO is a natural approach to accurate energy predictions for large molecular systems. The G2MS method predicts activation energies for ethylene + butadiene and ethylene + cyclopentadiene of 23.9 and 18.5 kcal/mol, respectively. The IMOMO(G2MS:MP2) method has been used to obtain accurate activation barriers for a number of Diels-Alder reactions, including the dimerization of butadiene where the calculated value of 23.5 kcal/mol is within 1 kcal/mol of two experimental values. For the addition of acrylic acid to 2,4-pentadienoic acid, a nearly quantitative agreement in the branching ratio for the product regio- and stereoisomers has been obtained. Calculations of the activation barriers for larger Diels-Alder reaction systems were performed, including the reaction of maleic anhydride with isoprene and 2-tert-butyl-1,3-butadiene, where the conformation of the reactant diene is found to be an important factor in determining the activation energy.