Cluster Theory for Open-Shell Molecules

Coupled Cluster Theory for Open-Shell Molecules 

Several researchers have recently devoted considerable effort to the derivation and efficient implementation of techniques based on spin-restricted reference determinants that reduce the computational discrepancy between closed- and open-shell systems. This emphasis on spin-restricted techniques has resulted in part from a bias toward reference wavefunctions that maintain the spin symmetry of the exact wavefunction (such as the ROHF determinant), but also because of the possible efficiency advantages of spin-restricted methods over unrestricted techniques. Thus, since the component molecular orbitals are constrained to have identical spatial parts for each spin function, it should be possible to construct the correlated wavefunction in a manner that takes advantage of this symmetry. 

It should be noted, however, that the use of a spin-symmetry-adapted determinant such as the ROHF wavefunction as a reference in a coupled cluster calculation does produce a spin-pure energy, but does not imply that the correlated wavefunction itself is an eigenfunction of as well. For the spin-orbital definition of T described here, spin contamination can still enter into the coupled cluster wavefunction through the nonlinear contributions of cluster operators to the amplitude equations, though the importance of this 

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