Dual-Level Methods for Electronic Structure Calculations of Potential Energy Functions That Use Quantum Mechanics as the Lower Level

This chapter overviews the status of three dual-level approaches for potential surface calculations that use quantum mechanics for both the upper and lower level. Three types of approach are singled out for discussion: SEC and SAC calculations, IMOMO calculations with harmonically capped correlated small systems, and dual-level direct dynamics. The scaling external correlation (SEC) and scaling all correlation (SAC) methods are semi-ab initio approaches to the calculation of bond energies and barrier heights for chemical reactions. The IMOMO calculations are very similar in spirit to QM/MM methods, but the lower level is quantum mechanical. Dual-level direct dynamics is a general technique for combining levels in dynamics calculations that include quantum mechanical tunneling contributions.