Multireference perturbation theory MRPT

When MCSCF wavefunctions are used as the reference, the most commonly used methods for recovering the electron correlation are multireference configuration interaction (MRCI) (15) and multireference perturbation theory (MRPT)... 

Because the contribution of the external orbitals to the correlation energy is neglected, this missing dynamic correlation can be essentially recovered in two ways multireference configuration interaction (MRCI) or multireference perturbation theory (MRPT) approaches [53]. 

As an alternative for the variational methods, one can also apply multireference perturbation theory (MRPT) to calculate magnetic interactions. In principle, this type of calculations makes it possible to treat larger systems with more unpaired electrons. Among the many different implementations, two schemes are especially popular in the field of magnetic interactions CASPT2 [13] and NEVPT2 [14, 15], which will be shortly overviewed here. 

A computationally less expensive alternative to coupled-cluster theory is provided by multireference perturbation theory (MRPT). The currently most widely used form of MRPT in f element chemistry is second-order Complete Active Space Perturbation Theory (CASPT2) [79,80], and we shall focus on this method in this section. 

Figure 1. The shape of the potential curve for nitrogen in a correlation-consistent polarized double-zeta basis set is presented for the variational 2-RDM method as well as (a) single-reference coupled cluster, (b) multireference second-order perturbation theory (MRPT) and single-double configuration interaction (MRCl), and full configuration interaction (FCl) wavefunction methods. The symbol 2-RDM indicates that the potential curve was shifted by the difference between the 2-RDM and CCSD(T) energies at equilibrium.

Equilibrium Bond Distance and the Harmonic Frequency for N2 from the 2-RDM Method with 2-Positivity (DQG) Conditions Compared with Their Values from Coupled-Cluster Singles-Doubles with Perturbative Triples (CCD(T)), Multireference Second-Order Perturbation Theory (MRPT), Multireference Configuration Interaction with Single-Double Excitations (MRCI), and Full Configuration Interaction (FCI)". 

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