1,721,051 research outputs found
Transdimensional Markov Chain Monte Carlo Using Hyperplane Inflation in Locally Nested Spaces - Rapporto tecnico n.2-2006 - Dipartimento di Statistica, Probabilità e Statistiche Applicate - Sapienza Università di Roma
No full textWe present some new results that extend the geometric approach to trans- dimensional Markov chain Monte Carlo simulations originally proposed in Petris and Tardella (2003). These provide a black-box method to generate a sample from a Markov chain with a prescribed stationary distribution on a disjoint union of Euclidean spaces not necessarily of the same dimension. The only requirement is that the support spaces of different dimensions have to be locally nested and the corresponding densities of the target distribu- tion have to be known up to a normalizing constant. Empirical evidence of effectiveness of the proposed method is provided by a controlled experiment of variable selection in a general regression context as well as by an original approach to mixture of normal models.
Rapporto Tecnico #4, Dipartimento di Satistica, Probabilità e Statistiche Applicate, Università "La Sapienza" - Rom
A New Strategy for Simulating From Mixture Distributions With Applications to Bayesian Model Selection
No full textWe present a method of generating random vectors from a distribution having an absolutely continuous component and a discrete component. The method is then extended to more general mixture distributions that arise quite naturally when dealing with nested models within a Bayesian framework. The main idea is to transform the mixture distribution of interest into an absolutely continuous one, in a way that does not require the explicit calculation of the relative weights of the various components of the mixture. For nested models, the proposed method represents a simple alternative to Reversible Jump MCMC schemes. Its distinguishing features are the absence of a proposal step to reduce/increase the dimension of the current space and the fact that in order to assess the convergence of the chain, one can use all the standard tools available for MCMC on a space of fixed dimension
KEY WORDS: Bayesian inference; model averaging; Markov chain Monte Carlo.
QUADERNO DI DIPARTIMENTO n. 24 - Serie A - Ricerch
A geometric approach to transdimensional MCMC.
No full textThe authors present theoretical results that show how one can simulate a mixture distribution whose components live in subspaces of different dimension by reformulating the problem in such a way that observations may be drawn from an auxiliary continuous distribution on the largest subspace and then transformed in an appropriate fashion. Motivated by the importance of enlarging the set of available Markov chain Monte Carlo (MCMC) techniques, the authors show how their results can be fruitfully employed in problems such as model selection (or averaging) of nested models, or regeneration of Markov chains for evaluating standard deviations of estimated expectations derived from MCMC simulations
Theoretical investigations of atmospheric species relevant for the search of high-energy density materials
No full textIonization of carbonyl sulphide/disulphur monoxide mixtures in atmospheric gases: A theoretical study of the formation of S3O+ ions
No full textA theoretical study performed at B3LYP and CCSD(T) level of theory of the reaction of formation Of S3O+ in ionized carbonyl sulphide/disulpbur monoxide mixtures is reported here. The interaction of carbonyl sulphide with disulphur monoxide cation leads to three different [CS3O2](+) isomers, essentially formed by a S3O+ cation solvated by a CO molecule. The S3O+ cation is present in the three structures cis, trans and ring, showed also by the isolated S3O+ species. The complex with the ring S3O+ unit is the most stable, but a direct ath to its formation starting from OCS and S2O+ has not been found in the potential energy surface Of [CS3O2]+. This species can be obtained from the isomerization of the complexes containing the cis or trans S3O+ unit, overcoming relatively low barrier heights
Direct Experimental Observation of CS2OH
No full textThe first experimental detection of CS2OH is reported. CS2OH was observed for about one microsecond after its formation, as an intact isolated species in the gas phase. It was generated by electron transfer to the CS2OH+ ion, prepared in the source of a multisector mass spectrometer by suitable ion-molecule reactions. The vertical formation process allowed characterization of CS2OH by structural analysis of CS2OH+. Theoretical calculations were performed at the B3LYP/6-311 + G(2dp) and CCSD(T)/aug-cc-pVTZ//B3LYP/6-311 + G(2d,p) levels of theory. The computed structure and stability of CS2OH and CS2OH+ as well as the energetics of the involved processes satisfactorily fit with the experimental results
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