Moller-Plesset principle

Theory. Usually we do not solve the fundamental equations directly. We use a theory, for example, Har-tree-Fock theory [3], Moller-Plesset perturbation theory [4], coupled-cluster theory [5], Kohn s [6, 7], Newton s [8], or Schlessinger s [9] variational principle for scattering amplitudes, the quasiclassical trajectory method [10], the trajectory surface hopping method [11], classical S-matrix theory [12], the close-coupling approximation... 

Theoretical studies, like first-principles calculations, grand canonical ensemble Monte Carlo (GCMC) simulations, second order Moller-Plesset perturbation theory (MP2) calculations and density functional theory (DFT) calculations, have been utilized to investigate optimal structures and their properties. Combined experimental and theoretical data provide a window to the plan of design of these network structures and lead to a new direction to investigate porous networks. 

To explain some principles, we focus here on the most simple of these approaches, the Moller-Plesset perturbation theory to second order (MP2). In the conventional, canonical MO-based formulation, the closed-shell MP2 correlation energy is given by... 

First principles methods need less parametrisation, since the hamiltonian is built up from more fundamental principles, although simplifications and even some level of parametrisation remains necessary for practical use. For instance, in the case of DFT, that point is particularly evident in the construction of the exchange-correlation energy functional, which is not known a priori for an arbitrary molecular system. ° In the case of Hartree-Fock, the correlation effects are not considered and post-Hartree Fock adjustments are necessary to include such effects, for instance in the form of perturbative expansion terms of the type derived in the Moller-Plesset perturbation theory of second order (MP2). ... 

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