Search

A system associated with several number of weak interactions supports numerous number of stable structures within a narrow range of energy. Often, a deterministic search method fails to locate the global minimum geometry as well as important local minimum isomers for such systems. Therefore, in this work, the stochastic search technique, namely parallel tempering, has been executed on the quantum chemical surface of the CNO(‐)(H2O)n system for n=1–8 to generate global minimum as well as several number of local minimum isomers. IR spectrum can act as the fingerprint property for such system to be identified. Thus, IR spectroscopic features have also been included in this work. Vertical detachment energy has also been calculated to obtain clear information about number of water molecules in several spheres around the central anion. In addition, in a real experimental scenario, not only the global but also the local minimum isomers play an important role in determining the average value of a particular physically observable property. Therefore, the relative conformational populations have been determined for all the evaluated structures for the temperature range between 20K and 400K. Further to understand the phase change behavior, the configurational heat capacities have also been calculated for different sizes. [ABSTRACT FROM AUTHOR]

Complementarity of position and momentum representation is at the heart of quantum mechanics. Dissociation of diatomic molecule can be investigated and controlled in conventional coordinate space as well as in momentum space. Our objective in this article is to have the control over the dissociation process irrespective of the representation. We have applied Simulated Annealing (SA) technique for the optimisation of the field, where the effective field strength is kept sufficiently low. The optimisation is carried out in both momentum and position representation on with completely different objective function but leading to similar result. [ABSTRACT FROM AUTHOR]

The problem of identifying low‐energy structures of Cl2−H2On (n = 1‐6) was investigated, and the evaluation of important properties like heat capacity, solvation energy, and vertical detachment energy for each of the clusters was carried out. The problem was handled at two different theoretical levels. First, an adequately chosen empirical potential energy surface was used to account for the major interactions between the constituents of the cluster studied. Once the surface was chosen, the Parallel tempering algorithm was employed to search out the low‐energy critical points on this surface, which gave geometries at this level. To refine the structures further, these pre‐optimized structures were used as inputs for quantum chemical evaluation to complete the final refinement. To check whether the structures found were reasonable, sensitive properties like heat capacity, solvation energy, and vertical detachment energy were calculated. Then, an effort was made to understand and explain the variations in these properties with change in the cluster size. To understand the process of cluster formation further, thermodynamic aspects like △H (298.15 K), △G (298.15 K), and heat capacity (Cv) changes were also evaluated. Infrared spectroscopic features were also studied to see whether the introduction of the Cl2− ion caused reasonable shifts compared to a pure water cluster. [ABSTRACT FROM AUTHOR]

We present a stochastic optimisation procedure using simulated annealing for studying conformational changes in (MgF 2) n clusters. The pathways for transformation between stable geometries through a transition state is studied for cluster sizes n=3−12. The work uses an objective function which exploits the sign of the eigenvalues of the Hessian matrix to locate transition states. Once it is located for a specific size it is examined if one minimum geometry can be converted to another through the transition state. The potential energy surface for the system is searched using the stochastic optimisation procedure of simulated annealing. The ruggedness of the surface makes the choice of the optimiser an ideal one. To complete the results the progress of the pathway is depicted as the function of the path coordinates at intermediate geometries. [ABSTRACT FROM AUTHOR]

In this work, parallel tempering has been used to explore the quantum chemical surface of SCN(HO) system for n=1−8. The stretching frequencies and more specifically the shifts in -O-H stretching frequencies in this type of systems can serve as the fingerprint of a particular geometry. That is why, stretching frequencies of the water molecules have been evaluated and the changes in their values with successive addition of water molecules are also observed. The general trend of getting more and more red shifted peaks is also seen. Although, in some cases, the trend is slightly deviated, which is also mentioned and logical explanations have been given. Using this strategy several number of local structures along with the global one can also be located, as in a real experimental scenario, where finite temperature is present, any of the observable properties should not be guided by only the global minimum but should be an averaged out value contributed by the local structures along with the global one for a particular size of the cluster. Keeping this important fact in mind, the conformational populations of different isomers for a particular size have also been investigated for temperatures upto 300 K at 1 atm. pressure. [ABSTRACT FROM AUTHOR]

Clusters can exhibit enormous structural diversities and in most cases with the increase in size, the number of conformers that can be supported by the potential energy surface describing the system also increases at a rapid rate. In the present work, this issue has been addressed from the angle of finding out transformation pathways or the so-called reaction paths for interconversion among isomers in (MgO)n clusters, where ‘n’ denotes the size. Clusters with ten different values of ‘n’ starting from 6 and ending at 16 have been studied. For each of these sizes, effort has been made to identify the transition state geometry which connects two minimum configurations. Structural similarities or dissimilarities between the various conformers have been discussed so as to see if any basic unit gets repeated with increase in size of the clusters. The entire study has been conducted using the stochastic optimisation principle of simulated annealing and an adequately defined objective function which can uniquely guide the search towards a first-order saddle point or the transition state on a potential energy surface. [ABSTRACT FROM AUTHOR]

In this communication, we would like to discuss the advantages of adaptive mutation simulated annealing (AMSA) over standard simulated annealing (SA) in studying the Coulombic explosion of (CO2)n2+ clusters for n = 20–68, where ‘n’ is the size of the cluster. We have demonstrated how AMSA itself can overcome the predicaments which can arise in conventional SA and carry out the search for better results by adapting the parameters (only when needed) dynamically during the simulations so that the search process can come out of high energy basins and not go astray for better exploration and convergence, respectively. This technique also has in-built properties for getting more than one minimum in a single run. For a (CO2)n2+ cluster system we have found the critical limit to be n = 43, above which the attractive forces between individual units become greater in value than that of the large repulsive forces and the clusters stay intact as the energetically favoured isomers. This result is in good concurrence with earlier studies. Moreover, we have studied the fragmentation patterns for the entire size range and we have found fission type fragmentation as the favoured mechanism nearly for all sizes. [ABSTRACT FROM AUTHOR]

In this work we obtained global as well as local structures of Br2(−)(H2O)n clusters for n = 2 to 6 followed by the study of IR-spectral features and thermochemistry for the structures. The way adopted by us to obtain structures is not the conventional one used in most cases. Here we at first generated excellent quality pre-optimized structures by exploring the suitable empirical potential energy surface using stochastic optimizer simulated annealing. These structures are then further refined using quantum chemical calculations to obtain the final structures, and spectral and thermodynamic features. We clearly showed that our approach results in very quick and better convergence which reduces the computational cost and obviously using the strategy we are able to get one [i.e. global] or more than one [i.e. global and local(s)] energetically lower structures than those which are already reported for a given cluster size. Moreover, IR-spectral results and the evolutionary trends in interaction energy, solvation energy and vertical detachment energy for global structures of each size have also been presented to establish the utility of the procedure employed. [ABSTRACT FROM AUTHOR]

In this communication, we would like to discuss the advantages of adaptive mutation simulated annealing (AMSA) over standard simulated annealing (SA) in studying the Coulombic explosion of (CO 2 ) n 2+ clusters for n = 20-68, where 'n' is the size of the cluster. We have demonstrated how AMSA itself can overcome the predicaments which can arise in conventional SA and carry out the search for better results by adapting the parameters (only when needed) dynamically during the simulations so that the search process can come out of high energy basins and not go astray for better exploration and convergence, respectively. This technique also has in-built properties for getting more than one minimum in a single run. For a (CO 2 ) n 2+ cluster system we have found the critical limit to be n = 43, above which the attractive forces between individual units become greater in value than that of the large repulsive forces and the clusters stay intact as the energetically favoured isomers. This result is in good concurrence with earlier studies. Moreover, we have studied the fragmentation patterns for the entire size range and we have found fission type fragmentation as the favoured mechanism nearly for all sizes.

In this work we obtained global as well as local structures of Br2((-))(H2O)n clusters for n = 2 to 6 followed by the study of IR-spectral features and thermochemistry for the structures. The way adopted by us to obtain structures is not the conventional one used in most cases. Here we at first generated excellent quality pre-optimized structures by exploring the suitable empirical potential energy surface using stochastic optimizer simulated annealing. These structures are then further refined using quantum chemical calculations to obtain the final structures, and spectral and thermodynamic features. We clearly showed that our approach results in very quick and better convergence which reduces the computational cost and obviously using the strategy we are able to get one [i.e. global] or more than one [i.e. global and local(s)] energetically lower structures than those which are already reported for a given cluster size. Moreover, IR-spectral results and the evolutionary trends in interaction energy, solvation energy and vertical detachment energy for global structures of each size have also been presented to establish the utility of the procedure employed.