Multireference second-order perturbation

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)". 

Levine BG, Coe JD, Martinez TJ (2008) Optimizing conical intersections without derivative coupling vectors application to multistate multireference second-order perturbation theory (MS-CASPT2). J Phys Chem B 112 405... 

The methods of choice must be adequate for manifolds of electronic states that are localized around a lanthanide ion in a solid host. The combination of a solid environment, a heavy element, and 4/, 5d, and other open-shells, demands the consideration of the effects of the solid host, the use of relativistic Hamiltonians up to spin-orbit coupling, the correct treatment of static and dynamic correlation, and handling large manifolds of quasi-degenerate excited states. We decided to use embedded-cluster wavefunction theory-based (EC-WFT) methods, with a two-component relativistic Hamiltonian to be used in two-steps, a multi-configurational variational treatment of static correlation, and a multireference second-order perturbation theory treatment of dynamic correlation. 

Several different versions of second-order perturbation theory for multireference wave functions have been implemented but the one currently in widest use is probably the CASPT2 method. This method was developed by Roos and co-workers in Lund, Sweden, and it is available in their MOLCAS package of computer programs. 

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

The second-order perturbation calculations following the CAS-SCF treatments previously described are intended to supply the deficiency of correlation, if any, included in the latter. To compute the energy corrections to be added to the variational zero-order values obtained by diagonalizing the active-space Hamiltonian matrices (see tables 3a and 3b of section 2.2), we have performed two different kinds of multireference perturbation Cl treatments, both of them being based on improvements of the general CIPSI algorithm (26] recently developed by the Ferrara-Pisa group[27,28,29,30]. 

CASPT2 Second order perturbation theory on top of a CASSCF wave function yp n5 Cheapest for multireference states... 

Many-body Brillouin-Wigner second-order perturbation theory using a multireference formulation an application to bond breaking in the diatomic hydrides BH and FH Molecular Physics 104, 2367 (2006)... 

Multireference second-order Brillouin-Wigner perturbation theory Molecular Physics 102,701 (2004)... 

---