ON THE IMPORTANCE OF CORE POLARIZATION IN HEAVY POST-D ELEMENTS - A PSEUDOPOTENTIAL CALIBRATION STUDY FOR X(2)H(6) (X=SI, GE, SN, PB)

Recently published scalar-relativistic energy-adjusted pseudopotential s have been employed, in valence ab-initio calculations using extended basis sets, for the determination of geometries, atomization energies and rotational barriers of the X(2)H(6) (X = Si, Ge, Sn, Pb) compound s. While the importance of relativistic effects is not unexpected (the X-X bond length, r(XX), changes by -14 pm for X = Pb), the significan t impact of core polarization for the heavy post-d elements (Delta r(X X) = -5 pm for X = Pb) comes as a surprise. Comparison against all-ele ctron calculations, including core-valence correlation, for Ge2H6 prov es the frozen-core and core-polarization parts of our pseudopotential to be accurate to similar to 2 and 1 pm, respectively, for r(XX).