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Volume 136, Issue 20, 28 May 2012
Complexes of the benzenium ion () with N2 or CO2 have been studied by explicitly correlated coupled clustertheory at the CCSD(T)-F12x (x = a, b) level [T. B. Adler et al., J. Chem. Phys.127, 221106 (2007)10.1063/1.2817618] and the double-hybrid density functional B2PLYP-D [T. Schwabe and S. Grimme, Phys. Chem. Chem. Phys.9, 3397 (2007)10.1039/b704725h]. Improved harmonic vibrational wavenumbers for have been obtained by CCSD(T*)-F12a calculations with the VTZ-F12 basis set. Combining them with previous B2PLYP-D anharmonic contributions we arrive at anharmonic wavenumbers which are in excellent agreement with recent experimental data from p-H2 matrix isolation IR spectroscopy[M. Bahou et al., J. Chem. Phys.136, 154304 (2012)10.1063/1.3703502]. The energetically most favourable conformer of ·N2 shows a π-bonded structure similar to ·Rg (Rg = Ne, Ar) [P. Botschwina and R. Oswald, J. Phys. Chem. A115, 13664 (2011)10.1021/jp207905t] with De ≈ 870 cm−1. For ·CO2, a slightly lower energy is calculated for a conformer with the CO2 ligand lying in the ring-plane of the moiety (De ≈ 1508 cm−1). It may be discriminated from other conformers through a strong band predicted at 1218 cm−1, red-shifted by 21 cm−1 from the corresponding band of free .
Communication: Phase transitions, criticality, and three-phase coexistence in constrained cell models136(2012); http://dx.doi.org/10.1063/1.4725768View Description Hide Description
In simulation studies of fluid-solid transitions, the solid phase is usually modeled as a constrained system in which each particle is confined to move in a single Wigner-Seitz cell. The constrained cell model has been used in the determination of fluid-solid coexistence via thermodynamic integration and other techniques. In the present work, the phase diagram of such a constrained system of Lennard-Jones particles is determined from constant-pressure simulations. The pressure-density isotherms exhibit inflection points which are interpreted as the mechanical stability limit of the solid phase. The phase diagram of the constrained system contains a critical and a triple point. The temperature and pressure at the critical and the triple point are both higher than those of the unconstrained system due to the reduction in the entropy caused by the single occupancy constraint.