Search results for "Spin contamination"

Accurate Prediction of Hyperfine Coupling Tensors for Main Group Elements Using a Unitary Group Based Rigorously Spin-Adapted Coupled-Cluster Theory

2019

We present the development of a perturbative triples correction scheme for the previously reported unitary group based spin-adapted combinatoric open-shell coupled-cluster (CC) singles and doubles (COS-CCSD) approach and report on the applications of the newly developed method, termed "COS-CCSD(T)", to the calculation of hyperfine coupling (HFC) tensors for radicals consisting of hydrogen, second- and third-row elements. The COS-CCSD(T) method involves a single noniterative step with [Formula: see text] scaling of the computational cost for the calculation of triples corrections to the energy. The key feature of this development is the use of spatial semicanonical orbitals generated from st…

Perturbative calculation of spin-orbit splittings using the equation-of-motion ionization-potential coupled-cluster ansatz.

2008

Spin-orbit splittings for (2)Pi states are calculated within coupled-cluster (CC) theory via first-order degenerate perturbation theory. Using the equation-of-motion CC variant for ionization potentials (EOMIP-CC), the two components of the considered (2)Pi state are treated in a balanced way by generating both radical states via annihilation of one electron out of the CC wave function of the corresponding anion. We report on the implementation of the described approach within the CC singles and doubles approximation. To ensure computational efficiency, an atomic mean-field approximation for the spin-orbit integrals is used, resulting in a formulation in terms of one-electron transition-den…

A Discussion of Some Problems Associated with the Quantum Mechanical Treatment of Open-Shell Molecules

2003

Spin-restricted open-shell coupled-cluster theory for excited states

2000

Using a linear-response approach, the recently introduced spin-restricted coupled-cluster (SR-CC) theory is extended to the treatment of excited states of high-spin open-shell molecules. Explicit equations are given within the usual singles and doubles approximation and our implementation (within an existing spin–orbital code) is described. It is shown that in SR-CC theory, due to spin constraints, the spin-expectation value for the excited states calculated as corresponding energy derivatives always corresponds to the exact value. In addition, the SR-CC singles and doubles (SR-CCSD) approach is extended to include also the so-called pseudotriple excitations (best described as double excita…