Open-shell systems investigated with Monte Carlo configuration interaction

We investigate the open‐shell systems of the methyl radical, the allyl radical and molecular oxygen using Monte Carlo configuration interaction. We look at whether modifying Monte Carlo configuration interaction to use the increased flexibility of unrestricted Hartree‐Fock orbitals offers any benefits in the description of these systems by the resulting compact wavefunctions. The expectation of the total spin squared when using Slater determinants is calculated to investigate if pure spin states can be evolved by Monte Carlo configuration interaction and how this might be accelerated. We consider the multireference character of these open‐shell systems. To this end we demonstrate how a previously introduced way [J. P. Coe and M. J. Paterson, J. Chem. Theory Comput. 11, 4189 (2015)] of viewing a multireference indicator of P.‐O. Löwdin [P.‐O. Löwdin, Phys. Rev. 97, 1474 (1955)] in terms of the spatial entanglement can be generalized to open‐shell systems. Open‐shell systems may present challenges for electronic structure methods. Monte Carlo configuration interaction (MCCI) is investigated on such systems. This method is adapted to use unrestricted Hartree‐Fock orbitals and calculate expectations of the total spin squared. Pure spin states are seen to be approached in the evolution of the MCCI wavefunction despite starting with substantial spin contamination. A way to view a multireference indicator in terms of the spatial entanglement is generalized to open‐shell systems.