In electronic structure theory, accurate solutions require a balanced treatment of both static (non-dynamic) and dynamic correlation. Nowadays, it is necessary to resort to multi-reference methods for correctly handling both types of correlation, however, these techniques are often expensive and demand prior knowledge of the system. On the other hand, single-reference correlation methods are well-established for dynamic correlation, but are unsatisfactory for systems with static correlation. In this talk, a new method is presented capable of achieving dynamic and static correlation even in those difficult cases in which both types of correlation are equally present.<br/><br/>The starting-point is a determinant wavefunction formed with natural orbitals obtained from a new interacting-pair model. The latter leads to a natural orbital functional named PNOF7 capable of recovering the complete intra-pair, but only the static inter-pair correlation. Using the solution of the natural orbital functional, two new energy functionals are defined for both dynamic (Edyn) and static (Esta) correlation. Edyn is derived from a modified second-order Moller-Plesset perturbation theory (MP2), while Esta is obtained from the static component of the PNOF7. Double counting is avoided by introducing the amount of static and dynamic correlation in each orbital as a function of its occupation. The total energy is represented by the sum Ehf+Edyn+Esta, where Ehf is the Hartree-Fock energy obtained with natural orbitals. The resulting working formulas allow for static and dynamic correlation to be achieved in one shot, as is the case in the standard single-reference perturbation theory.<br/><br/>The new procedure called NOF-MP2 is extraordinarily simple, has fifth order formal scaling of computational cost, and the property of size-consistency, essential for applying the method to extended systems. It is applied successfully to the homolytic dissociation of a selected set of diatomic molecules that encompass very dissimilar interactions ranging from weak to strong bonds, thus sweeping a wide range of interelectronic interactions and correlation regimes. The values obtained are in outstanding agreement with the experimental data.

NOF-MP2: Global method for electron correlation

255th National Meeting (2018)

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