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Multireference systems, dynamical correlation

In summary, it can be said that multireference Cl methods provide a balanced description of static and dynamic correlation effects. This comes at the cost of a considerably increased computational demand compared to single-reference methods. Nevertheless, a multireference approach is inevitable for systems with a genuine multiconfigurational character such as symmetric transition metal compounds. [Pg.318]

The non-relativistic CASPT2 method developed by Anderson et al. [11,12] is one of the most familiar multireference approaches. It is well established and has been applied to a large number of molecular systems with the non- or quasi-relativistic approaches. Because the CASPT2 method treats dynamic correlation effects pertur-batively, it is less expensive than the multireference Cl (MRCI) method. The... [Pg.158]

The main restriction of the methods discussed in the main part of this chapter is the assumption of a single dominant configuration. This requirement is not met for systems with strong non-dynamical correlation effects such as biradicals (e.g. in bond-breaking situations) or transition metal compounds. In this case the reference function must be a multireference expansion. The fixed-amplitude approximation lends itself very well to a multireference extension of the formalism, as demonstrated first by Ten-no who devised an F12-based internally contracted geminal correction to the multireference MP2 (MRMP2) method. A closely related approach was considered by Torheyden and Valeev, who proposed a generalized perturbative correction to arbitrary wavefunctions and which they applied to a MRCI wavefunction. A spin-free formulation was reported recently.The approach was also used by Booth et al. to get a basis set limit estimate for their full Cl quantum Monte Carlo method, which can also be seen as an approach to tackle systems with multireference character. [Pg.58]

Hamiltonian proposed by Muller and Plesset gives rise to a very successful and efficient method to treat electron correlation effects in systems that can be described by a single reference wave function. However, for a multireference wave function the Moller-Plesset division can no longer be made and an alternative choice of B(0> is needed. One such scheme is the Complete Active Space See-ond-Order Perturbation Theory (CASPT2) developed by Anderson and Roos [3, 4], We will briefly resume the most important definitions of the theory one is referred to the original articles for a more extensive description of the method. The reference wave function is a CASSCF wave function that accounts for the largest part of the non-dynamical electron correlation. The zeroth-order Hamiltonian is defined as follows and reduces to the Moller-Plesset operator in the limit of zero active orbitals ... [Pg.230]

Further developments in F12 theory mainly address three directions (1) Combination with strategies to treat large systems with wavefunction methods, (2) balanced treatment of dynamic and non-dynamic electron correlation in multireference methods, and (3) relativistic effects. [Pg.57]


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