We describe a new multireference perturbation algorithm for ab initio electronic structure calculations, based on a generalized valence bond (GVB) reference system, a local version of second-order Moller-Plesset perturbation theory (LMP2), and pseudospectral (PS) numerical methods. This PS-GVB-LMP2 algorithm is shown to have a computational scaling of approximately N-3 with basis set size N, and is readily applicable to medium to large size molecules using workstations with relatively modest memory and disk storage. Furthermore, the PS-GVB-LMP2 method is applicable to an arbitrary molecule in an automated fashion (although specific protocols for resonance interactions must be incorporated) and hence constitutes a well-defined model chemistry, in contrast to some alternative multireference methodologies. A calculation on the alanine dipeptide using the cc-pVTZ(-f) basis set (338 basis functions total) is presented as an example. We then apply the method to the calculation of 36 conformational energy differences assembled by Halgren and co-workers [J. Comput. Chem. 16, 1483 (1995)], where we obtain uniformly good agreement (better than 0.4 kcal/mole) between theory and experiment for all test cases but one, for which it appears as though the experimental measurement is less accurate than the theory. In contrast, quadratic configuration interaction QCISD(T) calculations are, surprisingly, shown to fail badly on one test case, methyl vinyl ether, for which the calculated energy difference is 2.5 kcal/mole and the experimental value is 1.15 kcal/mole. We hypothesize that single reference methods sometimes have difficulties describing multireference character due to low lying excited states in carbon-carbon pi bonds.