FIRST-ORDER RELATIVISTIC CORRECTIONS TO MP2 ENERGY FROM STANDARD GRADIENT CODES - COMPARISON WITH RESULTS FROM DENSITY-FUNCTIONAL THEORY

The evaluation of the first-order scalar relativistic corrections to M P2 energy based on either direct perturbation theory or the mass-veloc ity and Darwin terms is discussed. In a basis set of Levy-Leblond spin ors the one- and two-electron matrix elements of the relativistic Hami ltonian can be decomposed into a nonrelativistic part and a relativist ic perturbation. Thus, a program capable of calculating nonrelativisti c energy gradients can be used to calculate the cross-term between rel ativity and correlation. The method has been applied to selected close d-shell atoms (He, Be, Ne, and Ar) and molecules (CuH, AgH, and AuH). The calculated equilibrium distances and harmonic frequencies were com pared with results from first-order relativistic density functional ca lculations. It was found that the cross-term is not the origin of the nonadditivity of relativistic and correlation effects. (C) 1998 John W iley & Sons, Inc.