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 standard (nonrelativistic) contracted basis set can produce erroneous results for elements beyond the first row elements. The generation of DKH contracted cc-pVXZ(X=D,T,Q,5) basis sets for H, He, B–Ne, Al–Ar, and Ga–Br is discussed. The effect of DKH at the Hartree–Fock level on the bond distances, vibrational frequencies, and total dissociation energies for CF4, SiH4, SiF4, and Br2CO is discussed. It is suggested that the predominant effect of the scalar relativistic correction on the total dissociation energy can be calculated at the Hartree–Fock level if an adequate basis set is used.