INTEGRATED MOLECULAR-ORBITAL METHOD WITH HARMONIC CAP FOR MOLECULAR FORCES AND ITS APPLICATION TO GEOMETRY OPTIMIZATION AND THE CALCULATIONOF VIBRATIONAL FREQUENCIES

When applied to large systems, the integrated-levels strategy has been demonstrated to be a good approximation for the calculation of energy differences with an accuracy comparable to the accuracy that would be obtained by a single-level calculation at the higher level with only a part of its computational expense. In the present paper we introduce a modification to the original integrated method, namely, an additive harmonic term that allows a very simple and general extension of the integrated methods to the calculation of forces (gradients), optimizat ion of geometries, and calculation of harmonic vibrational frequencies . The new method, called integrated molecular orbital method with harm onic cap (IMOHC), has been successfully tested by calculating the C-H bond distances, vibrational frequencies, and bond energies in the etha ne molecule. The method should serve as an efficient method for creati ng dual-level methods of the QM:MM and IMOMO variety, and the forces c an be used to drive classical (Car-Parrinello) or semiclassical (varia tional transition-state theory with semiclassical tunneling) direct dy namics calculations.