Valence and correlated basis sets for the first-row transition atoms from Sc to Zn

Contracted Gaussian-type function sets are developed for the valence 4s and 3d orbitals and for correlated functions of the first-row transition atoms from Sc to Zn. A segmented contraction scheme is used for its compactness and computational efficiency. The contraction coefficients and exponents of the valence and correlated sets are determined by minimizing the differenc es from weighted averages of accurate atomic natural orbitals for the 4s(2) 3d(n-2) and 4s(1)3d(n-1) atomic states. The new basis sets give a well-bala nced description for these configurations at the Hartree-Fock level and yie ld more than 97% of the atomic correlation energies predicted by accurate n atural orbitals of the same size. Molecular tests of the present basis func tions are performed for the FeCO molecule at complete-active-space self-con sistent-field and at single and double excitation configuration interaction levels. The present sets show an accuracy similar to that of the averaged atomic natural orbital sets in spite of 3-5 times shorter computation time in the generation of two-electron integrals.