Pseudospectral localized Mo/ller–Plesset methods: Theory and calculation of conformational energies

We have developed an algorithm based upon pseudospectral ab initio electronic structure methods for evaluating correlation energies via the localized Mo/ller–Plesset methodology of Pulay and Saebo. Even for small molecules (∼20 atoms) CPU times are diminished by a factor of ∼10 compared to canonical MP2 timings for Gaussian 92 and the scaling is reduced from N4−N5 in conventional methods to ∼N3. We have tested the accuracy of the method by calculating conformational energy differences for 36 small molecules for which experimental data exists, using the Dunning cc-pVTZ correlation consistent basis set. After removing 6 test cases on the grounds of unreliability of the experimental data, an average deviation with experiment of 0.18 kcal/mol between theory and experiment is obtained, with a maximum deviation of ∼0.55 kcal/mol. This performance is significantly better than that obtained previously with a smaller basis set via canonical MP2; it is also superior to the results of gradient corrected density functional theory.