Dirac-Hartree-Fock Perturbation Calculation of Magnetic Shielding Using the External Field-Dependent Restricted Magnetic Balance Condition

A formulism for four-component Dirac-Hartree-Fock perturbation theory using the restricted magnetic balance (RMB) condition with gauge-including atomic orbitals (GIAOs) is derived at an ab initio level. In this formulation, the zeroth-order Dirac-Hartree-Fock equations are subject only to the usual restricted kinetic balance (RKB) condition, while the external field-dependent RMB condition is introduced in the calculation of the first-order magnetically perturbed orbitals. The magnetic shielding can be divided into a paramagnetic part and a diamagnetic part, as for nonrelativistic (NR) shielding. The present theory is applied to the calculation of hydrogen halide and molecular halogen shieldings.