Configuration interaction spin contamination

Well-defined variational (Cl-type see Configuration Interaction), perturbational (MPn see M0ller-Plesset Perturbation Theory), and coupled cluster (CC see Coupled-cluster Theory) techniques have all been employed to determine anharmonic force fields. Important conclusions of these studies include (1) Near equilibrium, the correlation energy is a low-order function of the bond distances,even a linear approximation is meaningful(2) For open-shell species, spin contamination can significantly deteriorate results if a... 

Field (CASSCF) Second-order Perturbation Theory (CAS-PT2) Configuration Interaction Core-Valence Correlation Effects Coupled-cluster Theory Experimental Data Evaluation and Quality Control G2 Theory Heats of Formation Isoelectronic Isogyric Reactions M0ller-Plesset Perturbation Theory Numerical Hartree-Fock Methods for Molecules r 12-Dependent Wavefunctions Relativistic Theory and Applications Spectroscopy Computational Methods Spin Contamination Transition Metals Applications,... 

AMI Basis Sets Correlation Consistent Sets Complete Active Space Self-consistent Field (CASSCF) Second-order Perturbation Theory (CASPT2) Configuration Interaction Coupled-cluster Theory Density Functional Theory (DFT), Hartree-Fock (HF) and the Self-consistent Field Diradicals Electronic Wavefunctions Analysis G2 Theory M0ller-Plesset Perturbation Theory Natural Bond Orbital Methods Spin Contamination. 

Benchmark Studies on Small Molecules Configuration Interaction Gradient Theory Green s Functions and Propagators for Chemistry Molecular Magnetic Properties Mpl-ler-Plesset Perturbation Theory ru-Dependent Wavefunc-tions Spin Contamination. 

Thus, the main relativistic effects are (1) the radical contraction and energetic stabilization of the s and p orbitals which in turn induce the radial expansion and energetic destabilization of the outer d and f orbitals, and (2) the well-known spin-orbit splitting. These effects will be pronounced upon going from As to Sb to Bi. Associated with effect (1), it is interesting to note that the Bi atom has a tendency to form compounds in which Bi is trivalent with the 6s 6p valence configuration. For this tendency of the 6s electron pair to remain formally unoxidized in bismuth compounds (i.e. core-like nature of the 6s electrons), the term inert pair effect or nonhybridization effect has been often used for a reasonable explanation. In this context, the relatively inert 4s pair of the As atom (compared with the 5s pair of Sb) may be ascribed to the stabilization due to the d-block contraction , rather than effect (1) . On the other hand, effect (2) plays an important role in the electronic and spectroscopic properties of atoms and molecules especially in the open-shell states. It not only splits the electronic states but also mixes the states which would not mix in the absence of spin-orbit interaction. As an example, it was calculated that even the ground state ( 2 " ) of Bij is 25% contaminated by Hg. In the Pauli Hamiltonian approximation there is one more relativistic effect called the Dawin term. This will tend to counteract partially the mass-velocity effect. 

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