Quasirelativistic methods effects

The quasirelativistic (QR) PP of Hay and Wadt [61] use two-component wave functions, but the Hamiltonian includes the Darwin and mass-velocity terms and omits the spin-orbit effects. The latter are then included via the perturbation operator after the wave functions have been obtained. The advantage of die method is the possibility to calculate quite economically rather large systems. The method is implemented in the commercial system Gaussian 98 It has extensively been applied to calculations of transition-element and actinide systems [62],... 

Table 3 presents relativistic effects on several properties calculated as the difference (A) obtained in calculations which included the quasirelativistic correction, and corresponding calculations that excluded the correction, and used Hartree-Fock-Slater core orbitals rather than Dirac-Slater. The method finds significant relativistic Pt-C bond shortening, and little effect on the CO bond. The effect on adsorption energy is dramatic. Eads increases by about 50% when relativity is included. There is also an increase in the Pt-C force constant and frequency. The shortened Pt-C bond results in an increase in CO frequency through a wall effect, a Pauli repulsion effect. Ref. 34 ascribed the anomalously small shift in CO frequency from gas phase to adsorbed on Pt to the relativistic effect. 

As the fully relativistic (four-component) calculations demand severe computational efforts, several quasirelativistic (two-component) approximations have been proposed in which only large components are treated explicitly. The approaches with perturbative treatment of relativistic effects [507] have also been developed in which a nonrelativistic wavefunction is used as reference. The Breit-Pauli (BP) approximation uses the perturbation theory up to the (p/mc) term and gives reasonable results in the first-order perturbation calculation. Unfortunately, this method cannot be used in... 

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