New approaches for quantum chemical calculations on very large molecules

This is the final report for a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Ab initio quantum chemistry has proven to be a valuable tool for understanding the electronic structure of small molecules containing 5--50 atoms. Calculations on larger systems have been prohibitive because of the forbidding scaling properties of the methods used. In the simplest traditional many-body method, second-order perturbation theory, the work scales as the fifth power of N, where N is proportional to the number of electrons in the system. New methods for the many-body problem, which scale as the third power of N and are well suited to utilize parallel computation, have been studied. The combination of these two developments enable the treatment of molecules in the range of 50--500 atoms with first principles methods. The ability to treat molecules containing 50--500 atoms with quantum chemistry techniques would open new vistas for theoretical chemistry. Large organic molecules of pharmaceutical interest and novel species such as fullerenes would be amenable to study. Inorganic complexes containing transition metals and actinides could be treated using realistic ligands. Important problems such as the chemistry at active sites in enzymes and the photochemistry involved in photosynthesis of chlorophyll could be studied. Catalytic reactions of hydrocarbons on zeolites and other supports of interest to the petrochemical and chemical industries could be examined