Ab initio pseudopotentials for orbital-free density functionals

– A method for producing pseudopotentials from first principles for use with orbitalfree density functionals is presented. In several metallic systems, the orbital-free approach reproduces the results of Kohn-Sham calculations with non-local pseudopotentials with considerably less computational effort. There has been a revitalization of interest in the use of orbital-free density functionals for electronic structure calculations [1]-[3], following the realization that extremely efficient ab initio molecular-dynamics methods, which scale almost linearly with system size, can be constructed in this way [4]-[9]. At the same time, there has been continued interest in finding functionals with which to represent the electron density at the metal/vacuum interface [10]. The central problem, in substituting an orbital-free (O-F) approach for the usual KohnSham (K-S) realization of density functional theory [11], is to find a representation of the kinetic energy. It has been shown how functionals which replicate the behaviour of the K-S functional in simple metallic systems, such as Na and Al [12], may be constructed using the linear [2], [3], [5], [10] and quadratic [3], [12] response functions of non-interacting electron gas, together with the classical Thomas-Fermi and von Weiszäcker functionals [1]. Solving the kinetic-energy problem does not, however, mean that full, first-principles O-F calculations on metals are immediately possible. The orbitals play a second role in ab initio K-S calculations, which is to allow a representation of non-locality in the pseudopotentials which are used in practical calculations where plane-wave basis sets are employed. Such pseudopotentials are constructed from all-electron calculations on the isolated atom [13]. Different potentials are required for atomic orbitals of each angular momentum and these are implemented in the condensed phase calculations by projecting out of the orbital wave functions the s, p, etc. components around each atom and applying the appropriate pseudopotential to each. In the O-F calculations, the wave functions are not represented, only the total density. Hence, local () While on leave of absence from Department of Chemistry, UCLA, 405 Hilgard Av., Los Angeles, CA 90095, USA.