Optimized virtual orbitals for relativistic calculations: an alternative approach to the basis set construction for correlation calculations†

Abstract

A recently proposed method for creating the optimized virtual orbital space (OVOS) is used in relativistic calculations. In the present implementation the starting point is mostly an uncontracted Gaussian primitive basis set. In the coupled cluster CCSD(T) electron correlation step, the space of virtual orbitals is reduced via OVOS to the size which corresponds to the well established HyPolX or PolX basis sets developed by Sadlej and his collaborators, or even smaller. This preserves the space of the occupied orbitals as represented in the uncontracted basis set and, at the same time, it leads to considerable savings of the computer time. The performance of OVOS is demonstrated in calculations of dipole moments and dipole polarizabilities of the series of the coinage metal (Me = Cu, Ag, and Au) anions and cations and the hydrides and fluorides, CuH, AgH, AuH, CuF, AgF and AuF. The method is particularly useful when combined with the higher order DKH (Douglas–Kroll–Hess) approach. OVOS allows reducing the space of the virtual orbitals obtained from the primitive basis set to a size tractable by the sophisticated coupled cluster calculations. The same primitive basis set and the corresponding space of virtual orbitals can be used within different relativistic Hamiltonians. Our approach also allows avoiding the necessity of contracting the primitive basis set specifically with any particular relativistic Hamiltonian. Even if we did extensive testing calculations mostly using the second-order DKH Hamiltonian, our approach is shown to be useful also with the spin-free infinite order two-component approach as defined by Barysz, Sadlej and Snijders. The OVOS technique can serve as an alternative to the contraction of the primitive Gaussian basis. †Dedicated to Professor Andrzej J. Sadlej on the occasion of his 65th birthday.