Relativistic corrections in single-point energy calculations and geometry optimizations can be computed:
- using perturbation theory based on the usual mass-velocity and one-electron Darwin terms (MVD1),
- using perturbation theory based on mass-velocity and one- and two-electron Darwin terms (MVD2),
- using the so-called direct perturbation theory approach (DPT) in second-order (DPT2),
- using higher-order direct perturbation theory (DPT4 with spin-orbit effects included, scalar-relativistic treatment at DPT6, not part of the public release),
- using a spin-free Dirac-Coulomb (sfDC) approach (not part of the public release),
- using the spin-free X2C-1e (SFX2C-1e) approach.
Relativistic corrections to first-order properties (dipole moments, quadrupole moments, and electric-field gradients) and second-order properties (static and dynamical polarizabilities, not part of the public release) can be computed at the DPT2 level. Relativistic corrections to analytic forces for geometry optimizations are available for the MVD schemes as well as for DPT2.
Note that a finite-nucleus model based on a Gaussian charge distribution is available for relativistic calculations.