Integrated molecular orbital method with harmonic cap for molecular forces and its application to geometry optimization and the calculation of 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 calculator 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), optimization of geometries, and calculation of harmonic vibrational frequencies. The new method, called integrated molecular orbital method with harmonic cap (IMOHC), has been successfully tested by calculating the C-H bond distances, vibrational frequencies, and bond energies in the ethane molecule. The method should serve as an efficient method for creating dual-level methods of the QM:MM and IMOMO variety, and the forces can be used to drive classical (Car-Parrinello) or semiclassical (variational transition-state theory with semiclassical tunneling) direct dynamics calculations.