Spin – orbit DFT with analytic gradients and applications to heavy element compounds

We have implemented the unrestricted DFT approach with one-electron spin – orbit operators in the massively parallel NWChem program. Also implemented is the analytic gradient in the DFT approach with spin – orbit interactions. The current capabilities include single-point calculations and geometry optimization. Vibrational frequencies can be calculated numerically from the analytically calculated gradients. The implementation is based on the spin – orbit interaction operator derived from the effective core potential approach. The exchange functionals used in the implementation are functionals derived for nonspin – orbit calculations, including GGA as well as hybrid functionals . Spin – orbit Hartree – Fock calculations can also be carried out. We have applied the spin – orbit DFT methods to the Uranyl aqua complexes. We have optimized the structures and calculated the vibrational frequencies of both ( UO

2

2+

) aq and ( UO

2+

) aq with and without spin – orbit effects. The effects of the spin – orbit interaction on the structures and frequencies of these two complexes are discussed. We also carried out calculations for Th

2

, and several low-lying electronic states are calculated. Our results indicate that, for open-shell systems, there are signifi cant effects due to the spin – orbit effects and the electronic confi gurations with and without spin – orbit interactions could change due to the occupation of orbitals of larger spin – orbit interactions.