# Equation of state, phonons, and lattice stability of ultra-fast warm dense matter

## TL;DR

This paper compares the equation of state, phonons, and lattice stability of ultra-fast warm dense matter using the NPA model and DFT/MD simulations, highlighting the NPA's efficiency at high temperatures and its insights into lattice stability.

## Contribution

The study introduces the NPA model as an efficient alternative to DFT+MD for high-temperature warm dense matter, providing detailed insights into lattice stability and ablation processes.

## Key findings

- Excellent agreement between NPA and DFT results for properties
- NPA effectively addresses temperatures above 8 eV where DFT+MD becomes prohibitive
- Finite-temperature exchange-correlation impacts pressure and electron density distribution

## Abstract

Using the two-temperature model for ultrafast matter (UFM), we compare the equation of state, pair-distribution functions $g(r)$, and phonons using the neutral pseudoatom (NPA) model with results from density-functional theory (DFT) codes and molecular-dynamics (MD) simulations for Al, Li and Na. The NPA approach uses state-dependent first-principles pseudopotentials from an `all-electron' DFT calculation with finite-$T$ XCF. It provides pair potentials, structure factors, the `bound' and `free' states, as well as a mean ionization $\bar{Z}$ unambiguously. These are not easily accessible {\it via} DFT+MD calculations which become prohibitive for $T/T_F$ exceeding $\sim 0.6$, where $T_F$ is the Fermi temperature. Hence, both DFT+MD and NPA methods can be compared up to $\sim 8$ eV, while higher $T$ can be addressed ${\it via}$ the NPA. The high-$T_e$ phonon calculations raise the question of UFM lattice stability and surface ablation in thin UFM samples. The ablation forces in a UFM slab are used to define an "ablation time" competing with phonon formation times in thin UFM samples. Excellent agreement for all properties is found between NPA and standard DFT codes, even for Li where a strongly non-local pseudopotential is used in DFT codes. The need to use pseudopotentials appropriate to the ionization state $\bar{Z}$ is emphasized. The effect of finite-$T$ exchange-correlation functional is illustrated via its effect on the pressure and the electron-density distribution at a nucleus.