Relativistic configuration interaction and coupled cluster methods using four-component spinors: Magnetic shielding constants of HX and CH3X (XÂ =Â F, Cl, Br, I)

A correlation method in the relativistic theory using four-component spinors is proposed for the calculations of magnetic shielding constants. The relativistic effects are included by solving Dirac-Fock equation, and the electron-correlation effects are included by the SDCI method and the CCSD method. Some improvement on the integral transformation algorithm and the use of the direct CI method were essential for a performance of this method. It is applied to the calculations of the magnetic shielding constants of hydrogen halides and methyl halides. For hydrogen halides the calculated values excellently reproduce the experimental values to within 0.8 ppm.