Multireference singles and doubles

Another class of methods uses more than one Slater determinant as the reference wave function. The methods used to describe electron correlation within these calculations are similar in some ways to the methods listed above. These methods include multiconfigurational self-consistent field (MCSCF), multireference single and double configuration interaction (MRDCI), and /V-clcctron valence state perturbation theory (NEVPT) methods.5... 

Modular FORTRAN programs for performing general ab initio multireference single- and double-excitation configuration interaction, averaged coupled-pair functional and linearized couple-cluster method calculations. Cray and other versions. 

Walch et performed CASSCF calculations involving 3088 configurations. At this level of theory, Ctj is unbound and does not show a minimum. The authors estimate that, to achieve a reasonable description within this type of approach, a multireference singles and doubles Cl based on the CASSCF function would involve 57 million configurations. The PLO corrections of Das and Jaffe are also insufficient to yield a bound molecule. 

MR-AQCC multireference averaged quadratic coupled-cluster MRCI multireference configuration interaction MRD-CI multireference single and double excitation Cl... 

All electron multireference single and double Cl calculations (Mavridis, A. Herrera, F. and Harrison, J.F., MSU, unpublished work) predict that the ground state of ScNHj is of Aj symmetry and that it is planar with the geometry... 

Keywords Configuration interaction (Cl) Full Cl Multireference single- and double-excitation Cl (MRD-CI) Condon rules Slater determinants Table Cl Direct Cl Spin-orbit couphng... 

Since its introduction by Buenker and Peyerimhoff in 1974 [8, 9], the multireference single- and double-excitation... 

Chwee TS, Szilva AB, Lindh R, Carter EA (2008) Linear scaling multireference singles and doubles configuration interaction. J Chem Phys 128 224106-1/9... 

H + CH2 CH3 This reaction has been the subject of several experimental [11] and theoretical studies [12]. The published theoretical work calculated portions of the hindered rotation barriers for different values of Rh-C Ihe distance of the attacking H from CH2. This calculation employed a tzp basis set [13] with an multireference singles and doubles configuration interaction (MRSDCI) wavefunction and was carried out with the COLUMBUS program system [14]. This work has now been supplemented by a much more extensive mapping of the hindered rotational potential with a less expensive dzp basis set [13], Tests show that Vtr is not sensitive to dzp or tzp basis set selection. 

Estimated limits based on large uncontracted even-tempered Gaussian calculations. The Cl results for oxygen are based on the estimated full Cl energy derived from large multireference single- and double-excitation calculations. 

ABSTRACT. The results of multireference singles and doubles Cl calculations of potential energy surfaces for hydrogen atom addition to O2, N2, and NO and recombination of OH -h O are discussed. The errors due to the use of externally contracted Cl and due to the neglect of correlation of O 2s and N 2s electrons are analyzed. Similarities and differences between the surfaces for the addition reactions are discussed. The calculated HN2 addition surface is used in a simple dynamical treatment (one-dimensional tunneling through an Eckart barrier) to estimate the lifetime of the HN2 species. The OH -h O recombination potential is found to exhibit complex features which require that electrostatic forces (dipole-quadrupole) and chemical forces be treated consistently. 

The calculations are CASSCF/multireference single and double excitation Cl (MRCI). The use of the MRCI method for computing potential energy surfaces is supported by full Cl studies as discussed by Bauschlicher, Langhoff, and Taylor in another article in this volume. The accuracy of this approach has also been demonstrated for reaction (2) by Walch [2], who showed that by systematically expanding the active space and basis set, the computed barrier height converged to the currently accepted value [14]. 

QCISD(T) = quadratic Cl including single, double, and triple excitations, UCCD(ST) = coupled-cluster doubles method based on the unrestricted Hartree-Fock method and corrected for single and triple replacements, MC SCF = multiconfiguration SCF, MRD Cl = multireference singles and doubles Cl, MBPT= many-body perturbation theory, SD Cl = singles and doubles Cl. 

CSF = configurational spin functions GUGA = graphical unitary group approach MRSDCl = multireference singles and doubles configuration interaction RCl = relativistic Cl RECP = relativistic effective core potential. 

Chwee TS, Carter EA. Valence Excited States in Large Molecules via Local Multireference Singles and Doubles Configuration Interaction. J Chem Theory Comput. 2010 7 103-111. 

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