Biomedical subjects

Silvano Romano

Publications and source records attributed to Silvano Romano.

6 recordsLinked to original sources

PubMed2006

Computer simulation study of a simple cubatic mesogenic lattice model.

Over the last , the possible existence of a cubatic mesophase, possessing cubic orientational order (i.e., along three mutually orthogonal axes) but no translational one, has been addressed theoretically, and predicted in some cases, where the investigated interaction models involved hard-core repulsion only; on the other hand, no experimental realizations of such a phase are known at the time being. The present paper addresses a very simple cubatic mesogenic lattice model, involving continuous interactions; we consider particles possessing Oh symmetry, whose centers of mass are associated with a three-dimensional simple-cubic lattice; the pair potential is taken to be isotropic in orientation space, and restricted to nearest neighboring sites; let the two orthonormal triads {uj, j=1, 2, 3} and {vk, k=1, 2, 3} define orientations of a pair of interacting particles, and let fjk=vj.uk. The interaction model studied here is defined by the simplest nontrivial (quartic) polynomial in the scalar products f jk, consistent with the assumed symmetry and favoring orientational order; it is, so to speak, the cubatic counterpart of the Lebwohl-Lasher model for uniaxial nematics. The model was investigated by mean field theory and Monte Carlo simulation, and found to produce a low-temperature cubatically ordered phase, undergoing a first order transition to the isotropic phase at higher temperature; the mean field treatment yielded results in reasonable qualitative agreement with simulation.

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PubMed2006

Topological transitions in two-dimensional lattice spin models.

A family of classical statistical-mechanical spin models, by now extensively studied in the literature, involves two-component unit vectors, associated with a two-dimensional lattice, with pair potentials restricted to nearest neighbors and possessing 0(2) symmetry--i.e., defined by some function of the scalar product between the two interacting spins; these studies often show the existence of a topological phase transition. We show here that, for a wide class of interaction models of the above type, available mathematical results entail the existence of a topological (Berezinskiĭ-Kosterlitz-Thouless-like) transition, as well as a rigorous lower bound on the transition temperature.

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PubMed2005

Bifurcation analysis and computer simulation of biaxial liquid crystals.

We extend the analysis of a mean-field model for biaxial liquid crystals recently proposed by Sonnet et al. [Phys. Rev. E 67, 061701 (2003)]. In particular, we perform a bifurcation analysis of the equilibrium equations and derive the complete phase diagram. We show that two order parameters suffice to label all equilibrium phases, though they exhibit different bifurcation patterns. A Monte Carlo simulation study is performed as well, confirming qualitatively the predictions of this analysis.

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PubMed2005

Minimal coupling model of the biaxial nematic phase.

A minimal coupling model exhibiting isotropic, uniaxial, and biaxial nematic phases is analyzed in detail and its relation to existing models known in the literature is clarified. Its intrinsic symmetry properties are exploited to restrict the relevant ranges of coupling constants. Further on, properties of the model are thoroughly investigated by means of bifurcation theory as proposed by Kayser and Raveché [Phys. Rev. A 17, 2067 (1978)] and Mulder [Phys. Rev. A 39, 360 (1989)]. As a first step toward this goal, the bifurcation theory is applied to a general formulation of density functional theory in terms of direct correlation functions. On a general formal level, the theory is then analyzed to show that the bifurcation points from the reference, high-symmetry equilibrium phase to a low-symmetry structure depend only on the properties of the one-particle distribution function and the direct pair correlation function of the reference phase. The character of the bifurcation (whether spinodal, critical, tricritical, isolated Landau point, etc.) depends, in addition, on a few higher-order direct correlation functions. Explicit analytical results are derived for the case when only the leading L=2 terms of the potential (mean-field analysis) or of the direct pair correlation function expansion in the symmetry-adapted basis are retained. Formulas are compared with the numerical calculations for the mean-field, momentum L=2 potential model, in which case they are exact. In particular, bifurcations from the isotropic and uniaxial nematic to the biaxial nematic phases are discussed. The possibility of the recently reported nematic uniaxial-nematic biaxial tricritical point [A. M. Sonnet, E. G. Virga, and G. E. Durand, Phys. Rev. E 67, 061701 (2003)] is analyzed as well.

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PubMed2005

The three corrugated surfaces of 1,4-divinyltetramethylene diradical intermediates and their connections to 1,2-divinylcyclobutane, 4-vinylcyclohexene, 1,5-cyclooctadiene, and two butadienes.

The three potential energy surfaces of the trans-trans, cis-trans, and cis-cis divinyltetramethylene diradicals have been located with DFT calculations at the BPW91/6-311+G levels. The three surfaces account well for the experimental results reported for the thermolysis of optically active trans-1,2-divinylcyclobutane and optically active and deuterated 4-vinylcyclohexene. The surfaces account also for the outcome of the dimerization of butadiene and the thermolysis of cis,cis-1,5-cyclooctadiene. The three diradical intermediates are connected to the cyclization and dissociation products through conformations that are explored fully here.

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PubMed2003

A bispericyclic transition structure allows for efficient relief of antiaromaticity enhancing reactivity and endo stereoselectivity in the dimerization of the fleeting cyclopentadienone.

B3LYP/6-31G calculations account for the enhanced reactivity and endo stereoselectivity in the dimerization of the fleeting antiaromatic cyclopentadienone. Secondary orbital interactions promote endo stereoselectivity and a full merging of 4+2 and 2+4 allowed paths in an endo bispericyclic transition structure. Electrostatic effects increase reactivity and selectivity but the driving force to enhanced reactivity is the loss of antiaromaticity in the dimerization TSs while enhanced selectivity derives from the more efficient relief of antiaromaticity in the bispericyclic array.

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