With our electron force field, we find that we obtain correct geometries for a wide range of hydrocarbons, particularly ones with a rigid carbon core and outwardly-directed hydrogens. Using a formulation that contains only pairwise interactions between the nuclei and electrons in the system, we are able to describe not only bonds, but reasonable energy differences between different hydrocarbon conformers. Multiple bonds are described as -like structures, and carbon radicals are properly planar. We are able to describe ionic compounds like LiH and correctly, as well as multicenter compounds like .
Further work is needed though. Lone pairs are poorly described ( and HF and Ne are unstable), and multiple bonds and radicals are too diffuse, causing carbon-carbon bonds to be too strong; this suggests we need a better way of describing electrons. Electron sizes tend to be too variable, particularly in C-H bonds. eFF also underestimates the strength of covalent atom-centered bonds, i.e., is underbound while is not. On a larger scope, we would like to add correlation, resonance/delocalization, and proper Fermi-Dirac statistics to eFF.
In simulating warm dense deuterium dynamics, we looked at temperature dissociation and ionization of in a region where it might be expected to have mixed covalent and metallic character. We obtained an equation of state and shock Hugoniot curve that was in agreement with most experiments [51,52,53] and path integral Monte Carlo calculations . We found no evidence for a plasma phase transition in the temperature and density range considered, contrary to some predictions , but consistent with recent path-integral Monte Carlo studies .
In simulating the Auger dynamics of small hydrocarbons, we found that after a core electron was removed, a valence electron transferred to reoccupy the core within a few femtoseconds, followed by additional valence electrons ejecting and the molecule fragmenting over tens of femtoseconds. The time scales were on the same scale as those observed experimentally . When core electrons were removed from small hydrocarbons, we observed selective bond breaking and secondary electron ejection; in contrast, core ionization inside a diamondoid particle caused secondary electrons to be released but rapidly recombined with the core, with no bonds broken. That bond cleavage occurs only near the surface and only near excited sites may help to explain the precision of surface etching observed in photon and electron stimulated desorption. Finally, we offer the intriguing possibility that in some systems, such as methane, formation of the two-hole state may be nonconcerted, so that the secondary electron leaves significantly after the core hole is filled, when bond breaking may have already occurred.
With eFF, we can compute the energy and forces in systems containing a thousand electrons in less than a second. We have shown that compounds containing atoms from hydrogen through carbon are reasonably well described, and the accuracy is sufficient to make possible the simulation of matter at extreme conditions. Work is in progress to improve the accuracy and scope of eFF, and we hope that the formulation presented here, as well as its progenitors, will enable the simulation of a wide range of interesting excited electron chemistry on realistic systems.