CO2 packing polymorphism under pressure: Mechanism and thermodynamics of the I-III polymorphic transition

Abstract

In this work, we describe the thermodynamics and mechanism of CO polymorphic transitions under pressure from form I to form III combining standard molecular dynamics, well-tempered metadynamics, and committor analysis. We find that the phase transformation takes place through a concerted rearrangement of CO molecules, which unfolds via an anisotropic expansion of the CO supercell. Furthermore, at high pressures, we find that defected form I configurations are thermodynamically more stable with respect to form I without structural defects. Our computational approach shows the capability of simultaneously providing an extensive sampling of the configurational space, estimates of the thermodynamic stability, and a suitable description of a complex, collective polymorphic transition mechanism.

CO2 packing polymorphism under confinement in cylindrical nanopores

“…A complete description of the mathematical formulation of λ has been reported by Giberti et al 38 . Explorative WTMetaD simuations are carried out biasing two distinct formulations of the λ order parameter: λ I , tuned to capture the molecular arrangement of CO 2 form I, which we recently developed to investigate CO 2 polymorphism at high pressure 19 ; and λ B , based instead on the characteristic angles and local density of the most abundant ordered structure observed under confinement (configuration B in Table IV). For details on the parameters used to define both λ I and λ B refer to Table II.…”

“…Our work has a two-fold aim: on the one hand understanding phase behaviour of confined CO 2 is relevant due to its prominent role within the carbon cycle, and the surging needs for mitigating its emissions in atmosphere by implementing capture and storage technologies based on adsorption in porous solids [15][16][17][18] . On the other hand due to its modest structural complexity accompanied with a rich phase diagram, CO 2 represents a convenient model system to perform extensive sampling of polymorphic transitions at finite temperature 19 and gain insight on general aspects of molecular phase transitions under confinement.…”

Building maps in collective variable space

“…Block averaged ∆U (d), P ∆V (e), ∆H (f), and −T ∆S are reported as a function of the collective variables λI and λIII that were introduced and discussed at length in Ref. 35 . In all the maps a green line indicates the minimum free energy path for the I-III transition.…”

Computing transition rates for rare events: When Kramers theory meets the free-energy landscape

“…Building on the standard KT and the metadynamics [16,17] (metaD) framework, the rate is first expressed in terms of the height of the FE barrier measured along the MFEP. We then define an auxiliary measure of the configurational entropy in the metastable basin based on the reconstruction of the FE landscape obtained from metaD simulations [18].…”

“…Since its introduction in 1981 by Kushick and Karplus in the context of macromolecules [14], a number of methods have been proposed in the literature to estimate the configurational entropy of complex systems [14,15,26,27]. We consider here the definition of the FE difference between two metastable basins B i and B j , ∆F * ij , in terms of the probability distribution of the collective variables (CVs) along which the FE landscape is projected [18] to assess quantitatively the entropic contribution of the FE surface:…”