Nonadiabatic Computations—Where Theory Delivers

For the smallest quantum systems, comprising perhaps up to five particles, one can afford nof infroducing fhe separafion of fhe elecfronic and nuclear degrees of freedom, i.e., nof infroducing fhe BO approximation. For historical reasons such compulations of energy levels are usually referred fo as nonadiabafic fhough strictly speaking they should be called diabatic. 

For three- and four-particle systems, like and H2, sophisticated nonadia-batic computations have been performed wifh specialized techniques [3,20-27], These compulations can yield rovibronic energy levels whose accuracy is limited only by the Hamiltonian used for fheir evaluafion. Unlike a BO freafment, nona-diabatic compulations can distinguish befween cerfain specfroscopic characteristics of the different isotopologues. Nevertheless, while nonadiabatic computations yield energy levels in a quantitative way, the qualitative characterization of them is somewhat difficult. 

In a recent paper [23] we made an attempt to retain the notion of a PES in a nonadiabatic treatment. This was achieved by fixing fhe infernuclear separation in 

The fully nonadiabatic treatment of few-body systems have yielded very accurate energy levels and transition energies. At the limit of these calculations, when even QED effects are considered, the energies have not only internal consistency but are in almost full accord with the relevant results of measuremenfs. 

As to many-electron systems, corrections to the BO approximation can be obtained by means of a second-order contact transformation method [28]. This introduces two terms (a) the simple DBOC, which gives rise to a mass-dependent correction to the PES and (b) the considerably more difficult second-order (also called non-adiabatic) correction, which introduces coupling between electronic states and primarily results in corrections to the kinetic energy operator. In the most sophisticated first-principles treatments [17,34,35] allowance is made for non-adiabatic effects though further work is required to explore the best possible strategies for compufafion and utilization of this information. 

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Nonadiabatic Computations—Where Theory Delivers MARVEL—An Active Database Approach Electronic structure computations... 

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