Local spin from strongly orthogonal geminal wavefunctions

Abstract

Covalent bond dissociation is examined by three geminal-based theories. One approach (antisymmetrised product of strongly orthogonal geminals) assumes purely singlet geminals, while two others operate with mixtures of singlets and triplets (cf. restricted–unrestricted strongly orthogonal singlet-type geminals, hereafter RUSSG). The RUSSG is the wavefunction in the second method. It is spin contaminated. The spin contamination of RUSSG is projected out in a spin-purification step in the third method. Description of local (i.e. atomic) spin by geminal-based theories is examined. Prototype systems showing the deficiency of singlet coupling are taken as test cases. We find that the local spin of equilibrium structures is correctly described by purely singlet geminals. Triplet geminals are shown to be essential for the description of local spin when dissociating multiple bonds, or switching between two Lewis structures of the same molecule.

Keywords:

• geminal
• strong orthogonality
• singlet coupling
• spin unrestricted
• local spin

Acknowledgements

Thanks are expressed to Tamás Zoboki for useful discussion. The FCI results and DMs of SP–RUSSG were computed by an implementation of Olsen’s FCI algorithm [47], written by Zoltán Rolik (Budapest University of Technology and Economics).

Additional information

Funding

This work was partially supported by the Hungarian Scientific Research Fund [grant number OTKA K81588]; Vitaly Rassolov’s work is supported by the Chemistry Division of the NSF [grant number CHE-1213604], and with computational resources provided in part [grant number CHE-104862].