THE TAUTOMERS OF URACIL - A LOCAL CORRELATION TREATMENT

The relative energies of all six uracil tautomers have been determined at the MP4(SDQ)/6-31G** level, using both conventional correlation theory and the Local Correlation method. Geometries were optimized at the SCF/6-31G* level with offset forces. Comparison of our energies with energies from structures optimized at the SCF level supports the conclusion that offset forces are an advantageous alternative to correlated geometry optimization. The Local Correlation method compares very well with conventional Moller-Plesset theory, recovering at least 98.4% of the conventional correlated energy in all cases. More importantly, the relative energies also show good agreement with the conventional results, even for these delocalized systems. CPU timings show a substantial computational savings for the Local Correlation method over the conventional method. The results of the local method using Boys localization are compared with those using Pipek-Mezey localization. The dioxo tautomer (1) is predicted to be the most stable. The (1)-(3) and (1)-(4) energy differences are found to be within the bounds estimated from experimental work.