Single/double excitation configurational electron correlation

There is also a hierarchy of electron correlation procedures. The Hartree-Fock (HF) approximation neglects correlation of electrons with antiparallel spins. Increasing levels of accuracy of electron correlation treatment are achieved by Mpller-Plesset perturbation theory truncated at the second (MP2), third (MP3), or fourth (MP4) order. Further inclusion of electron correlation is achieved by methods such as quadratic configuration interaction with single, double, and (perturbatively calculated) triple excitations [QCISD(T)], and by the analogous coupled cluster theory [CCSD(T)] [8],... 

In order to get good agreement between CASSCF and experimental results, it is often necessary to include correlation between the active electrons and the other valence electrons.This type of correlation, which is called dynamic correlation, can be added to a CASSCF wave function by using the equivalent of MP2 perturbation theory. However, in an MP2 calculation on an MC wave function, rather than including double excitations from a single HF reference configuration, excitations from all of the configurations must be included. 

As discussed in Chapter 9, the VBCI method provides results that are at par with the BOVB method, the difference being that the electrons of the spectator orbitals are correlated too in the VBCI method. The wave function starts from a VBSCF wave function and augments it with subsequent local configuration interaction that can be restricted to single excitations (VBCIS level), or single and double excitations (VBCISD), or higher excitations. Here, we will consider only the VBCISD level, which is a good compromise between accuracy and cost efficiency. 

In order to make allowance for the influence of electron correlation on the probability of the / -decay-induced excitation of a molecule, let us use the configuration interaction method. We will consider the configurations that take account only of the single and double electron excitations into the virtual excited orbitals. For the latter we will use the orbitals obtained by the Huzinaga-Arnau method (see above). The wave function of the ground state of the parent molecule is... 

Two approaches to electron correlation that are widely used today for the studies of organic radical cations are Coupled Cluster (CC) calculations or the similar, but not identical, Quadratic Configuration Interaction (QCI) method with single and double excitations, often followed by CCSD(T) or QCISD(T) single point calculations with a larger basis set. These methods suffer to a much lesser extent from... 

---