ABSTRACT

Integrated methods describe different  parts of a large compound (molecule or free radical) with different theoretical approaches. The main goal is to reproduce the results of a high-level theoretical calculation for a large system by dividing it into two parts: a small “model system” or inner layer, which is calculated at the highest theoretical level possible, and the “rest of the system” or outer layer, which is calculated at a lower level. Therefore, they represent an interesting and economical alternative for the problem of large compounds and high-level calculations.

Based mainly on the experience from our group, the performance of integrated methods in describing different molecular properties will here be analyzed. For the sake of clarity, this review is divided into three sections. In the first, the stationary point (reactant, product, and saddle point) properties, such as geometry, harmonic vibrational frequencies and energy changes (reaction and activation), are analyzed. In the second, the performance of these methods in obtaining reliable standard enthalpies of formation of free radicals is studied. Finally, in the third, an analysis is given of the extension of the integrated method idea to reaction-path construction in a chemical reaction involving the breaking-forming of covalent bonds in large compounds, and thence the kinetic and dynamic treatment.

In general, integrated methods reproduce the properties obtained with the high- level method, correct the deficiencies of the lower-level method showing their effectiveness in correcting wrong potential energy surfaces, and represent a substantial saving in computational cost (memory and time) which is especially interesting for the study of large molecules.

Two integrated methods, IMOMO (integrated molecular orbital-molecular orbital) and IMOHC (integrated molecular orbital- molecular orbital with harmonic cap) were analyzed, and gas-phase hydrogen abstraction reactions in the ground electronic state were used as test reactions.