Parallel Douglas-Kroll energy and gradients in NWChem: Estimating scalar relativistic effects using Douglas-Kroll contracted basis sets

A parallel implementation of the spin-free one-electron Douglas-Kroll-Hess (DKH) Hamiltonian in NWChem is discussed. An efficient and accurate method to calculate DKH gradients is introduced. It is shown that the use of a sta ndard (nonrelativistic) contracted basis set can produce erroneous results for elements beyond the first row elements. The generation of DKH contracte d cc-pVXZ(X = D,T,Q,5) basis sets for H, He, B-Ne, Al-Ar, and Ga-Br is disc ussed. The effect of DKH at the Hartree-Fock level on the bond distances, v ibrational frequencies, and total dissociation energies for CF4, SiH4, SiF4 , and Br2CO is discussed. It is suggested that the predominant effect of th e scalar relativistic correction on the total dissociation energy can be ca lculated at the Hartree-Fock level if an adequate basis set is used. (C) 20 01 American Institute of Physics.