Constrained Parmeterization of Reduced Density Approximation of Kinetic Energy Functionals

Evaluation of forces in ab initio MD is greatly accelerated by orbital-free DFT, especially at finite temperature [1]. The recent achievement of a fully non-empirical constraint-based generalized gradient (GGA) functional for the Kohn-Sham KE $T_s[n]$ [2] brings to light the inherent limitations of GGAs. This motivates inclusion of higher-order derivatives in the form of reduced derivative approximation (RDA)[3] functionals. That, in turn, requires new functional forms and design criteria. RDA functionals are constrained further to produce a positive-definite, non-singular Pauli potential. We focus on designing a non-empirical constraint-based meta-GGA[3-5] functional with certain combinations of higher-order derivatives which avoid nuclear-site singularities to a specified order of gradient expansion. Here we report progress on this agenda.\\[4pt] [1] Phys.\ Rev.\ B \textbf{86}, 115101 (2012);\\[0pt] [2] Phys.\ Rev.\ B \textbf{88}, 161108(R) (2013);\\[0pt] [3] Phys.\ Rev.\ B \textbf{80}, 245120 (2009);\\[0pt] [4] Phys.\ Rev.\ B \textbf{75}, 155109 (2007);\\[0pt] [5] Nuc.\ Phys.\ A \textbf{445} 263 (1985)