In this article we test three kinds of trial wave functions for the calculation of vibrational excited states of molecules using quantum Monte Carlo. We begin our study with the basis set originally used by Bernu and co-workers and further modified by Acioli and Soares Neto. The second set tested was the simplified Morse oscillator-like with harmonic coupling (SMOL-HC) proposed by Brown et al. to study the vibrational spectra of C3. Finally we proposed a third basis set, based on the previous two. This basis set keeps the anharmonicity of the SMOL-HC basis but with well conditioned Hamiltonian and overlap matrices. The calculations were performed in the H2, H3+, and H2O molecules. The results indicate that the basis sets proposed in this work yield more accurate results with a smaller number of basis functions.
ABSTRACT:In this article we present a characterization of the vibrational spectrum of the H 5 ϩ molecule using the correlation function quantum Monte Carlo (CFQMC) method and a genetic algorithm study of the topology of the potential energy surface (PES) used. The vibrational modes associated with the H 3 ϩ -H 2 torsion and stretching possess very flat minima. As a consequence the fundamental frequencies corresponding to these modes are poorly described in the harmonic approximation. In our genetic algorithm study of the PES using Cartesian coordinates we have found some unexpected minima. A careful analysis shows that, in the curvilinear coordinates in which the potential is described, some of these minima have identical coordinates. Nevertheless, they represent nonequivalent molecular geometries. The vibrational frequencies obtained in this work are not in good agreement with available experimental data as well as other computational methods. It shows that our method of detailed analysis of PESs could reveal shortcomings introduced by the use of curvilinear coordinates.
ABSTRACT:In this work, we present calculations of the vibrational energy levels of the H 2 ϩ and H 3 ϩ systems using the correlation function quantum Monte Carlo (CFQMC) and normal model analysis. The classical results are a qualitative first approximation of the normal modes. The results of the CFQMC calculations show the importance of the quantum effects as well as anharmonicity in these systems.
The addition of C to HCN is of relevant interest in astrochemistry. We studied the pathways of this addition to produce CCCN and estimated its reaction rate using the Master Equation in the circumstellar environment. From the results of this study, it was possible to show that a different pathway in the Surface Potential Energy-PES can also be investigated. In a circumstellar envelop environment, with temperatures varying between 1000 K and 2000 K, the abundances of these species are favorable to this kind of addition, and our branching ratio for the rate constant showed that the new pathway is more favorable in comparison with other possibilities for this range of temperatures in this environment, and must be taken into account in any computation of the rate constant. Graphical Abstract Branching ratios of pathways involved in the C2 + HCN → CCCN+H addition, at a temperature range of 1000-2000 K.
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