**4. Summary**

We studied the phase behavior of two-dimensional systems of Janus-like particles on a triangular lattice. Here, we assumed that all, AA, AB, and BB, interactions are attractive. The AA interaction energy was fixed, while the AB and/or BB interaction energies were varied, and assumed to be less attractive than the AA interaction. We assumed that the particles can take on only six different orientations and that the interaction energy between a pair of nearest neighbors depends on their mutual orientations. Using the grand canonical Monte Carlo simulation method, we considered three series of systems with *uAB* = *uBB*, *uAB* = 0 and with *uBB* = 0.

We demonstrated that the phase behavior of all systems strongly depends on the stability of the high density zigzag (Z) phase. The stability of the Z phase is determined by the anisotropy of interactions and increases when the AB and/or BB attractions become weaker. As a consequence, in the systems with sufficiently strong anisotropy of interactions, the liquid phase does not appear, and the dilute fluid condenses directly into the zigzag ordered phase. The transition terminates in the tricritical point. At temperatures above the tricritical point, the disordered fluid undergoes a continuous transition into the zigzag phase.

When the AB and/or BB attraction increases, the stability of the zigzag phase becomes weaker, and its formation is possible only at sufficiently high densities and at sufficiently low temperatures. This means that the dilute phase condenses into the zigzag phase only at the temperatures lower than the critical end point temperature. At temperatures above the critical end point, the dilute phase condenses into the disordered liquid-like phase, and the transition terminates in the usual critical point.

The above scenario was found in all three considered series of systems. Whenever the critical point appears, the critical temperature increases when the attraction between AB and/or BB halves becomes stronger. In the particular series with *uAB* = *uBB* = *<sup>u</sup>*<sup>∗</sup>, the critical temperature went up to the value corresponding to the critical point of the uniform system when *u*<sup>∗</sup> = −1.0. In the series with *uAB* = 0 and with *uBB* = 0, the critical temperatures reached lower values when *uBB* or *uAB* went to −1.0.

In the case of the series with *uAB* = 0, the phase diagram topology remained the same for any *uBB* lower than about −0.116. Thus, the onset of the continuous order–disorder transition in the dense fluid meeting the bulk coexistence in the critical end point, *Tcep*(*uBB*), and *Tcep*(*uBB*) gradually increased when *uBB* is lowered. On the other hand, the series with *uBB* = 0 is expected to show different behavior, when *uAB* decreases. In this paper, we discussed only the systems with *uAB* ≥ −0.25.

In this series, the critical end point temperature, *Tcep*(*uAB*), is bound to go to zero for *uAB* = −0.5, since this particular system does not undergo any orientational order– disorder transition [20]. However, a further decrease of *uAB* below −0.5 means that the order–disorder transition reappears; however, now, this transition belongs to the universality class of the four-state Potts model. Therefore, it is expected that the continuous order–disorder transition should occur at sufficiently high densities and at sufficiently low temperatures. The onset of this transition is also expected to be located at the critical end point, *Tcep*(*uAB*).

Here, we recall the results obtained for symmetric mixtures [27,28], which show qualitatively the same changes in the phase diagram topology when the tendency toward demixing becomes weaker. In that case, the demixed fluid is an ordered state, and by lowering its stability, the same sequence of phase diagram topologies appears.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References and note**


#### *Article* **Can We Predict the Isosymmetric Phase Transition? Application of DFT Calculations to Study the Pressure Induced Transformation of Chlorothiazide**

**Łukasz Szeleszczuk 1,\*, Anna Helena Mazurek 2, Katarzyna Milcarz 1, Ewa Napiórkowska 1 and Dariusz Maciej Pisklak 1**


**Abstract:** Isosymmetric structural phase transition (IPT, type 0), in which there are no changes in the occupation of Wyckoff positions, the number of atoms in the unit cell, and the space group symmetry, is relatively uncommon. Chlorothiazide, a diuretic agen<sup>t</sup> with a secondary function as an antihypertensive, has been proven to undergo pressure-induced IPT of Form I to Form II at 4.2 GPa. For that reason, it has been chosen as a model compound in this study to determine if IPT can be predicted in silico using periodic DFT calculations. The transformation of Form II into Form I, occurring under decompression, was observed in geometry optimization calculations. However, the reverse transition was not detected, although the calculated differences in the DFT energies and thermodynamic parameters indicated that Form II should be more stable at increased pressure. Finally, the IPT was successfully simulated using ab initio molecular dynamics calculations.

**Keywords:** DFT; CASTEP; aiMD; ab initio molecular dynamics; phase transition; polymorphism
