Generalized Hartree-Fock with Non-perturbative Treatment of Strong Magnetic Field: Application to Molecular Spin Phase Transition*

In this work, we present a framework of ab initio variational approach
to effectively explore the spin phase space in the presence of a homogenous
magnetic field. Complex generalized Hartree-Fock (C-GHF) in the
spinor formalism is implemented with London orbitals. Recursive algorithms for
computing one- and two-electron integrals of London orbitals are also provided.
A Pauli matrix representation of the C-GHF is introduced to
separate spin contributions from scalar part of the Fock matrix. The spin phase
transition in the several molecular systems in the presence of a strong magnetic
field is investigated. Non-collinear spin configurations are observed during
the phase transition. The competing driving forces of exchange coupling and spin Zeeman effect have been shown to govern
the spin phase transition and transition rate. The energetic contributions of the
spin Zeeman, orbital Zeeman, and diamagnetic terms to the potential energy
surface are also analyzed.