Volume 138, Issue 22, 14 June 2013
Index of content:
Communication: Two-dimensional gas-phase coherent anti-Stokes Raman spectroscopy (2D-CARS): Simultaneous planar imaging and multiplex spectroscopy in a single laser shot138(2013); http://dx.doi.org/10.1063/1.4810876View Description Hide Description
Coherent anti-Stokes Raman spectroscopy (CARS) has been widely used as a powerful tool for chemical sensing, molecular dynamics measurements, and rovibrational spectroscopy since its development over 30 years ago, finding use in fields of study as diverse as combustion diagnostics, cell biology, plasma physics, and the standoff detection of explosives. The capability for acquiring resolved CARS spectra in multiple spatial dimensions within a single laser shot has been a long-standing goal for the study of dynamical processes, but has proven elusive because of both phase-matching and detection considerations. Here, by combining new phase matching and detection schemes with the high efficiency of femtosecond excitation of Raman coherences, we introduce a technique for single-shot two-dimensional (2D) spatial measurements of gas phase CARS spectra. We demonstrate a spectrometer enabling both 2D plane imaging and spectroscopy simultaneously, and present the instantaneous measurement of 15 000 spatially correlated rotational CARS spectra in N2 and air over a 2D field of 40 mm2.
Communication: Energy benchmarking with quantum Monte Carlo for water nano-droplets and bulk liquid water138(2013); http://dx.doi.org/10.1063/1.4810882View Description Hide Description
We show the feasibility of using quantum Monte Carlo (QMC) to compute benchmark energies for configuration samples of thermal-equilibrium water clusters and the bulk liquid containing up to 64 molecules. Evidence that the accuracy of these benchmarks approaches that of basis-set converged coupled-cluster calculations is noted. We illustrate the usefulness of the benchmarks by using them to analyze the errors of the popular BLYP approximation of density functional theory (DFT). The results indicate the possibility of using QMC as a routine tool for analyzing DFT errors for non-covalent bonding in many types of condensed-phase molecular system.
Communication: Full dimensional quantum rate coefficients and kinetic isotope effects from ring polymer molecular dynamics for a seven-atom reaction OH + CH4 → CH3 + H2O138(2013); http://dx.doi.org/10.1063/1.4811329View Description Hide Description
The kinetic isotope effect (KIE) of the seven-atom reactions OH + CH4 → CH3 + H2O and OH + CD4 → CD3 + HDO over the temperature range 200–1000 K is investigated using ring polymer molecular dynamics (RPMD) on a full-dimensional potential energy surface. A comparison of RPMD with previous theoretical results obtained using transition state theory shows that RPMD is a more reliable theoretical approach for systems with more than 6 atoms, which provides a predictable level of accuracy. We show that the success of RPMD is a direct result of its independence of the choice of transition state dividing surface, a feature that is not shared by any of the transition state theory-based methods. Our results demonstrate that RPMD is a prospective method for studies of KIEs for polyatomic reactions for which rigorous quantum mechanical calculations are currently impossible.
Communication: An accurate global potential energy surface for the OH + CO → H + CO2 reaction using neural networks138(2013); http://dx.doi.org/10.1063/1.4811109View Description Hide Description
We report a new global potential energy surface of the HOCO system based on the F12 correction of unrestricted coupled-cluster with singles doubles and approximative triples using the augmented correlation-consistent polarized valence triple-zeta basis set (UCCSD(T)-F12/AVTZ), fitted by using the neural networks. Quantum dynamics calculations confirmed the satisfactory convergence of surface with respect to the number of data points and fitting process. It is found that the total reaction probabilities and complex-formation probabilities obtained on the present surface differ considerably with those obtained on the potential energy surface recently reported by Li et al. [J. Chem. Phys.136, 041103 (Year: 2012)]10.1063/1.3680256. Various comparisons revealed that the present surface is substantially more accurate than that surface, representing the best available potential energy surface for this benchmark complex-forming four-atom system.
Communication: An inexpensive, variational, almost black-box, almost size-consistent correction to configuration interaction singles for valence excited states138(2013); http://dx.doi.org/10.1063/1.4809571View Description Hide Description
Configuration interaction singles (CIS) describe excited electronic states only qualitatively and improvements are imperative as a means of recovering chemical accuracy. In particular, variational improvements would be ideal to account for state crossings and electronic relaxation. To accomplish such an objective, in this communication we present a new suite of algorithms, abbreviated VOO-CIS for variationally orbital optimized CIS. We show below that VOO-CIS yields a uniform improvement to CIS, rebalancing the energies of CT states versus non-CT states within the same framework. Furthermore, VOO-CIS finds energetic corrections for CT states that are even larger than those predicted by CIS(D). The computational cost of VOO-CIS depends strongly on the number of excited states requested (n), but otherwise should be proportional to the cost of CIS itself.