Complete active second order perturbation correction

Most of the description of metal-metal multiple bonds presented in this paper are based on calculations performed with the complete active space self-consistent-fleld (CASSCF) method [8] followed by second order perturbation theory, CASPT2 [9], The CASSCF method guarantees a qualitatively correct representation of the electronic structure, and the metal-metal multiple bonds. The subsequent... 

A many-body perturbation theory (MBPT) approach has been combined with the polarizable continuum model (PCM) of the electrostatic solvation. The first approximation called by authors the perturbation theory at energy level (PTE) consists of the solution of the PCM problem at the Hartree-Fock level to find the solvent reaction potential and the wavefunction for the calculation of the MBPT correction to the energy. In the second approximation, called the perturbation theory at the density matrix level only (PTD), the calculation of the reaction potential and electrostatic free energy is based on the MBPT corrected wavefunction for the isolated molecule. At the next approximation (perturbation theory at the energy and density matrix level, PTED), both the energy and the wave function are solvent reaction field and MBPT corrected. The self-consistent reaction field model has been also applied within the complete active space self-consistent field (CAS SCF) theory and the eomplete aetive space second-order perturbation theory. ... 

Additionally, one needs to remember that for a powerful tool such as pertnrbation theory, there is no obstacle to applying the multi-determinant reference function as the unperturbed function in perturbation calculus. Thus, similar to the SCF-HF and MP2 approaches, CASPT2 would be the second-order perturbation theory complete active space method - the perturbationaUy corrected CASSCF. 

Of course, a proper treatment would utilize multireference methods, but these are very time consuming and limited to model stmctures only [36, 88). Furthermore, dynamic electron correlation is also very important in these reactions so that, for instance, simple CASSCF (complete active space SCF) computations are meaningless, as they mostly include static correlation. Perturbative corrections to CASSCF wavefunctions such as CASPT2 (CAS perturbation theory of second order) are necessary however, multireference perturbation methods do not improve significantly the singlet-triplet energy separations of many biradicals as compared to CASSCF results [89]. 

---