We report the first wholly non-empirical generalized gradient approximation, non-interacting free energy functional for orbital-free density functional theory and use that new functional to provide forces for finite-temperature molecular dynamics simulations in the warm dense matter (WDM) regime The new functional provides good-to-excellent agreement with reference Kohn-Sham calculations under WDM conditions at a minuscule fraction of the computational cost of corresponding orbital-based simulations. PACS numbers: 31.15.E-, 71.15.Mb, 05.70.Ce, 65.40.G-Compared to ordinary condensed matter, the warm dense matter (WDM) regime [1, 2] poses experimental accessibility issues (e.g. inertial confinement fusion hohlraums [3]) that make computational characterization of WDM thermodynamics particularly significant. Current practice, for example Refs. [4, 5], is ab initio molecular dynamics (AIMD) with Born-Oppenheimer electronic forces on the ions from finite-T Kohn-Sham (KS) density functional [6-8] calculations. Computational costs for KS-AIMD scale no better than N 3 b per MD step, with N b the number of occupied KS orbitals. N b grows unfavorably with increasing T . KS-AIMD thus becomes prohibitively expensive at elevated T and path integral Monte Carlo (PIMC) simulations, which have comparable computational cost, come into play [2].A long-standing potential alternative to KS-DFT, orbital-free DFT (OFDFT), would scale linearly with system size. Use of OFDFT for WDM has been limited by clearly inadequate functionals, e.g. , for the non-interacting kinetic energy (KE) part T s of the free energy (though TF is, of course, the proper KS limit for high T and high material densities [5]). Ground-state two-point orbital-free KE functionals [10] are, unfortunately, of little utility for extension to WDM because those two-point functionals which treat different material phases equally well are both parameterized and introduce substantial extra computational complexity. Therefore we have focused on single-point functionals.Here we provide a new, non-empirical, generalized gradient approximation (GGA) T s functional and its associated entropy functional. They extend and rationalize the constraint-based, mildly empirically parameterized GGA functionals recently published [11]. We show that the new functionals make OFDFT-AIMD competitive with finite-T KS-AIMD calculations for accuracy and far faster. For deuterium in the WDM regime, the OFDFT AIMD and reference KS results agree well at intermediate T , 6 × 10 4 → 1.8 × 10 5 K. In the range

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The fundamental ideas for a nonlocal density functional theorycapable of reliably capturing van der Waals interactionswere already conceived in the 1990s. In 2004, a seminal paper introduced the first practical nonlocal exchange− correlation functional called vdW−DF, which has become widely successful and laid the foundation for much further research. However, since then, the functional form of vdW−DF has remained unchanged. Several successful modifications paired the original functional with different (local) exchange functionals to improve performance, and the successor vdW−DF2 also updated one internal parameter. Bringing together different insights from almost 2 decades of development and testing, we present the nextgeneration nonlocal correlation functional called vdW−DF3, in which we change the functional form while staying true to the original design philosophy. Although many popular functionals show good performance around the binding separation of van der Waals complexes, they often result in significant errors at larger separations. With vdW−DF3, we address this problem by taking advantage of a recently uncovered and largely unconstrained degree of freedom within the vdW−DF framework that can be constrained through empirical input, making our functional semiempirical. For two different parameterizations, we benchmark vdW−DF3 against a large set of well-studied test cases and compare our results with the most popular functionals, finding good performance in general for a wide array of systems and a significant improvement in accuracy at larger separations. Finally, we discuss the achievable performance within the current vdW−DF framework, the flexibility in functional design offered by vdW−DF3, as well as possible future directions for nonlocal van der Waals density functional theory.

High entropy alloys contain multiple elements in large proportions that make them prone to phase separation. These alloys generally have shallow enthalpy of mixing which makes the entropy contributions of similar magnitude. As a result, the phase stability of these alloys is equally dependent on enthalpy and entropy of mixing and understanding the individual contribution of thermodynamic properties is critical. In the overall vision of designing high entropy alloys, in this work, using density functional theory calculations, we elucidate the contributions of various entropies, i.e., vibrational, electronic and configurational towards the phase stability of binary alloys. We show that the contribution of electronic entropy is very small compared to the vibrational and configurational entropies, and does not play a significant role in the phase stability of alloys. The configurational and vibrational entropies can either destabilize or can collectively contribute to stabilize the solid solutions. As a result, even those systems that have negative mixing enthalpy can show phase instability, revealed as a miscibility gap; conversely, systems with positive mixing enthalpy can be phase stable due to entropic contributions. We suggest that including entropic contributions are critical in the development of theoretical framework for the computational prediction of stable, single-phase high entropy alloys that have shallow mixing enthalpies, unlike ordered intermetallics.