Evolution of quantum chemical calculations Beyond Hartree-Fock

7 Evolution of quantum chemical calculations Beyond Hartree-Fock 

In spite of all the evolution, there are still important problems for the calculation of intermolecular energies. Hartree-Fock (HF) methods use one-electron orbitals and therefore cannot account for those phenomena that depend on the simultaneous behavior of several electrons. Thus, HF energies may correctly represent the kinetic energies of electrons and the electrostatic effects between electrons and nuclei, but cannot take into account electron correlation. The results obtained at the limit of a complete (i.e. infinitely rich) basis set are called HF-limit energies and wavefunctions, the ideal best that can be obtained with one-electron orbitals. This intrinsic limitation forbids the treatment of dispersion energy, a crucial part of the intermolecular potential (see Chapter 4). Thus, for example, HF methods are intrinsically unsuitable for the calculation of the lattice energies of organic crystals. 

A powerful technique in quantum chemical manipulations is called perturbation theory. In many cases one has to deal with a hamiltonian operator for which the quantum chemical equations are too difficult or impossible to solve. A simpler hamiltonian may then be used to provide a zero-order solution, and then a perturbation operator is introduced, whose effect on the final results of the calculation can be obtained as a separate correction to the zero-order approximation. In Mpller-Plesset (MP) perturbation theory, the Fock operator is the zero-order hamiltonian (equation (c) in Box 3.1) and a Slater determinant is the zero-order wavefunction. The zero-order energy 

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