Application of the unitary group approach to evaluate spin density for configuration interaction calculations in a basis of S2 eigenfunctions

Funding information UK-EPSRC Grant ‘Attosecond Electron Dynamics in Molecular and Condensed Phase Systems’, Grant/Award Number: EP/I032517/1 Abstract We present an implementation of the spin-dependent unitary group approach to calculate spin densities for configuration interaction calculations in a basis of spin symmetry-adapted functions. Using S eigenfunctions helps to reduce the size of configuration space and is beneficial in studies of the systems where selection of states of specific spin symmetry is crucial. To achieve this, we combine the method to calculate U(n) generator matrix elements developed by Downward and Robb (Theor. Chim. Acta 1977, 46, 129) with the approach of Battle and Gould to calculate U(2n) generator matrix elements (Chem. Phys. Lett. 1993, 201, 284). We also compare and contrast the spin density formulated in terms of the spin-independent unitary generators arising from the group theory formalism and equivalent formulation of the spin density representation in terms of the oneand two-electron charge densities.