Perturbation theory complete active space second-order

RELATIVISTIC MULTIREFERENCE PERTURBATION THEORY COMPLETE ACTIVE-SPACE SECOND-ORDER PERTURBATION THEORY (CASPT2) WITH THE FOUR-COMPONENT DIRAC HAMILTONIAN... 

K. Hirao. Relativistic Multireference Perturbation Theory Complete Active-Space Second-Order Perturbation Theory (CASPT2) With The Four-Component Dirac Hamiltonian. In Radiation Induced Molecular Phenomena in Nucleic Adds, Volume 4 of Challenges and Advances in Computational Chemistry and Physics, p. 157-177. Springer, 2008. 

CASPT2 Complete active space second-order perturbation theory... 

We continue this section by pointing out that calculations exist that combine (PT) and MC methods. For example, methods such as complete active space, second order perturbation theory (CASPT2) and multi-configuration quasidegenerate perturbation theory MCODPT use a MC wave function as the reference. PT, generally to second order, is used to estimate the contribution from excited states that arise from excitation outside the original active space of the MC calculation. 

Our group has already studied all the fluoro-, chloro-and bromo-carbenes [11-16]. We used the complete active space self-consistent field (CASSCF) method as well as complete active space second-order perturbation theory (CASPT2) and multi-reference configuration interaction (MRCI) approaches to compute the geometries, force constants, and vibrational frequencies of the (singlet) X and A states as well as the (triplet) a states. Our theoretical studies of most of these carbenes were carried out specifically to complement LIF studies that were pursued in our laboratories by Kable et al. [6]. In addition to the determination of spectroscopic constants, the spectroscopic and theoretical studies considered dynamics on the A surfaces, i.e. whether photodissociation or internal conversion to the ground state would occur. 

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 complete active space second-order perturbation theory. 

M. Abe, T. Nakajima, K. Hirao. The relativistic complete active-space second-order perturbation theory with the foiu-component Dirac Hamiltonian. J. Chem. Phys., 125 (2006) 234110. 

CASPT2 Complete Active Space second-order Perturbation Theory... 

Our results for the vertical excitation energies (VEEs) of the four lowest-lying singlet excited electronic states are presented in Tables. 1, in comparison with previous calculations and experimental data. Previous calculations include the complete active space second-order perturbation theory (CASPT2) calculations of... 

Theoretical calculations were performed, initially with SCF-Xa-SW methods on a truncated model [16], and later with the complete active space self-consistent field (CASSCF) and mul-ticonfigurational complete active space second-order perturbation theory (CASPT2) methods on the full molecule [15]. The electronic structures from the two calculations were remarkably similar. The CASSCF/PT2 calculations predicted a single, dominant configuration (73%) with (a) (x) (x ) (a ) (8) (5 ). Although the formal bond order is 1.5, the effective bond order, which considers minor configurations that contribute to the ground-state wavefunction, is lower at 1.15. 

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. 

Complete Active Space Second-Order Perturbation Theory (CASPT2)... 

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