arXiv subjects

Marcin Kirsz

Publications and source records attributed to Marcin Kirsz.

3 recordsLinked to original sources

arXiv2026

Calculations of the Krypton Phase Diagram and Novel Plasticity

The phase diagram for Kr, as represented by the Tadah! two-body potential is shown to have face-centred cubic (fcc), hexahonal close packed (hcp), and body centred cubic (bcc) regions. It has been assembled by combining several methods: direct liquid--solid coexistence for the melt lines, Gibbs--Helmholtz integration and Clapeyron slopes for the bcc--fcc line, slab coexistence for the liquid--gas line, static zero-temperature relaxations for the crystals, and the quasiharmonic approximation for the low-temperature fcc--hcp windows. The bcc phase contains highly mobile ``greedy snake" defects, which suggests a reinterpretation of the melt-curve data: the anomaly observed may be due to the speckle method detecting the bcc-fcc boundary, not the melt curve. While pair potentials have limitations, comparison with a foundation MACE model shows that a more flexible machine-learned model does not necessarily improve matters if inappropriately trained.

cond-mat.mtrl-sci↗

arXiv2026

Frustrated supermolecules: the high-pressure phases of crystalline methane

Methane is the simplest hydrocarbon, yet it exhibits an extraordinarily complicated series of crystal phases. Notably, the non-plastic phases have large unit cells with nearly, but not quite cubic symmetry. Furthermore, although non-polar molecules interact very weakly, their reorganisation across phase transitions is very sluggish. Here, we demonstrate that these complex structures can be understood as simple packing of near-spherical supermolecular clusters of methane molecules: the departure from cubic symmetry arising from the non-spherical nature of the molecules. We use molecular dynamics based on density functional theory calculations to simulate the finite-temperature crystal structures of methane, finding that the complex Phase A is based around a 13-molecule regular icosahedron, with 8 additional molecules forming the 21-molecule unit cell. Similarly, Phase B is based on a body-centred cubic bcc packing of 17-molecule Z16 polyhedra, with the remaining 12 molecules per cell in tetrahedral interstices. We demonstrate that the favored intermolecular separation depends sensitively on molecular orientation, leading to hindered rotation and suppressed entropy. The structures are determined by a trade-off between efficient packing and entropy.

physics.comp-ph↗

arXiv2024

Understanding solid nitrogen through machine learning simulation

We construct a fast, transferable, general purpose, machine-learning interatomic potential suitable for large-scale simulations of $N_2$. The potential is trained only on high quality quantum chemical molecule-molecule interactions, no condensed phase information is used. The potential reproduces the experimental phase diagram including the melt curve and the molecular solid phases of nitrogen up to 10 GPa. This demonstrates that many-molecule interactions are unnecessary to explain the condensed phases of $N_2$. With increased pressure, transitions are observed from cubic ($α-N_2$), which optimises quadrupole-quadrupole interactions, through tetragonal ($γ-N_2$) which allows more efficient packing, through to monoclinic ($λ-N_2$) which packs still more efficiently. On heating, we obtain the hcp 3D rotor phase ($β-N_2$) and, at pressure, the cubic $δ-N_2$ phase which contains both 3D and 2D rotors, tetragonal $δ^\star-N_2$ phase with 2D rotors and the rhombohedral $ε-N_2$. Molecular dynamics demonstrates where these phases are indeed rotors, rather than frustrated order. The model does not support the existence of the wide range of bondlengths reported for the complex $ι-N_2$ phase. The thermodynamic transitions involve both shifts of molecular centres and rotations of molecules. We simulate these phase transitions between finding that the onset of rotation is rapid whereas motion of molecular centres is inhibited and the cause of the observed sluggishness of transitions. Routine density functional theory calculations give a similar picture to the potential.

physics.comp-ph↗