PICTURE CHANGE AND CALCULATIONS OF EXPECTATION VALUES IN APPROXIMATE RELATIVISTIC THEORIES

The effect of the so-called picture change on expectation values of on e-electron operators in approximate two(one)-component relativistic th eories is discussed. This effect is expected to be particularly large for operators which assume large values in the vicinity of heavy nucle i. The numerical results illustrating the picture change effect on ele ctric field gradients at nuclei have been obtained in the spin-free Pa uli and Douglas-Kroll approximations. It has been found that the pictu re change effect lowers the electric field gradient at I in HI by abou t 1 a.u. Very large picture change effect (-8 a.u.) has been calculate d for HAt. It is concluded that in accurate calculations of expectatio n values of operators involving high inverse powers of the electron-nu cleus distance the picture change, which accompanies the transformatio n of the Dirac (Dirac-Coulomb) equation to approximate two(one)-compon ent relativistic Hamiltonians, must be taken into account. (C) 1998 Jo hn Wiley & Sons, Inc.