Chemical application of diffusion quantum Monte Carlo

The diffusion quantum Monte Carlo (QMC) method gives a stochastic solution to the Schroedinger equation. This approach is receiving increasing attention in chemical applications as a result of its high accuracy. However, reducing statistical uncertainty remains a priority because chemical effects are often obtained as small differences of large numbers. As an example, the single-triplet splitting of the energy of the methylene molecule CH sub 2 is given. The QMC algorithm was implemented on the CYBER 205, first as a direct transcription of the algorithm running on the VAX 11/780, and second by explicitly writing vector code for all loops longer than a crossover length C. The speed of the codes relative to one another as a function of C, and relative to the VAX, are discussed. The computational time dependence obtained versus the number of basis functions is discussed and this is compared with that obtained from traditional quantum chemistry codes and that obtained from traditional computer architectures

Inelastic Collisions in an Ultracold quasi-2D Gas

We present a formalism for rigorous calculations of cross sections for inelastic and reactive collisions of ultracold atoms and molecules confined by laser fields in quasi-2D geometry. Our results show that the elastic-to-inelastic ratios of collision cross sections are enhanced in the presence of a laser confinement and that the threshold energy dependence of the collision cross sections can be tuned by varying the confinement strength and external magnetic fields. The enhancement of the elastic-to-inelastic ratios is inversely proportional to $\sqrt{\epsilon/\hbar \omega_0}$, where $\epsilon$ is the kinetic energy and $\omega_0$ is the oscillation frequency of the trapped particles in the confinement potential.Comment: 4 pages, 4 figure

Total angular momentum representation for atom-molecule collisions in electric fields

It is shown that the atom-molecule collision problem in the presence of an external electric field can be solved using the total angular momentum representation in the body-fixed coordinated frame, leading to a computationally efficient method for ab initio modeling of low-temperature scattering phenomena. Our calculations demonstrate rapid convergence of the cross sections for vibrational and Stark relaxation in He-CaD collisions with the number of total angular momentum states in the basis set, leading to a 5-100 fold increase in computational efficiency over the previously used methods based on the fully uncoupled space-fixed representation. These results open up the possibility of carrying out numerically converged quantum scattering calculations on a wide array of atom-molecule collisions and chemical reactions in the presence of electric fields.Comment: 19 pages, 3 figures, 1 tabl

Effects of System’s Limitations on the Accuracy of Measured Ultrasonic Correlated Signal

Within a highly attenuating material, it is often difficult to identify relevant target signals due to the system’s white noise that is elevated by high gains on a conventional ultrasonic system. Ultrasonic pulse compression technique resolves such problem. The ultrasonic pulse compression technique permits an ultrasonic system to operate with long transmitted pulses for an increased detection range, but without sacrificing the depth resolution by signal correlation. Moreover, the time integral involved in the cross-correlation further suppresses the system’s white noise, and hence it produces an improved signal-to-noise ratio (SNR)

All-electron quantum Monte Carlo calculations for the noble gas atoms He to Xe

We report all-electron variational and diffusion quantum Monte Carlo (VMC and DMC) calculations for the noble gas atoms He, Ne, Ar, Kr, and Xe. The calculations were performed using Slater-Jastrow wave functions with Hartree-Fock single-particle orbitals. The quality of both the optimized Jastrow factors and the nodal surfaces of the wave functions declines with increasing atomic number Z, but the DMC calculations are tractable and well behaved in all cases. We discuss the scaling of the computational cost of DMC calculations with Z

Quantum Monte Carlo study of the reaction: C1 + CH3OH -->CH2OH+ HCl

A theoretical study is reported of the Cl + CH{sub 3}OH {yields} CH{sub 2}OH + HCl reaction based on the diffusion Monte Carlo (DMC) variant of the quantum Monte Carlo method. Using a DMC trial function constructed as a product of Hartree-Fock and correlation functions, we have computed the barrier height, heat of reaction, atomization energies and heats of formation of reagents and products. The DMC heat of reaction, atomization energies, and heats of formation are found to agree with experiment to within the error bounds of computation and experiment. Moller-Plesset second order perturbation theory (MP2) and density functional theory, the latter in the B3LYP generalized gradient approximation, are found to overestimate the experimental heat of reaction. Intrinsic reaction coordinate calculations at the MP2 level of theory demonstrate that the reaction is predominantly direct, i.e., proceeds without formation of intermediates, which is consistent with a recent molecular beam experiment. The reaction barrier as determined from MP2 calculations is found to be 2.24 kcal/mol and by DMC it is computed to be 2.39(49) kcal/mol

Explicitly correlated trial wave functions in Quantum Monte Carlo calculations of excited states of Be and Be-

We present a new form of explicitly correlated wave function whose parameters are mainly linear, to circumvent the problem of the optimization of a large number of non-linear parameters usually encountered with basis sets of explicitly correlated wave functions. With this trial wave function we succeeded in minimizing the energy instead of the variance of the local energy, as is more common in quantum Monte Carlo methods. We applied this wave function to the calculation of the energies of Be 3P (1s22p2) and Be- 4So (1s22p3) by variational and diffusion Monte Carlo methods. The results compare favorably with those obtained by different types of explicitly correlated trial wave functions already described in the literature. The energies obtained are improved with respect to the best variational ones found in literature, and within one standard deviation from the estimated non-relativistic limitsComment: 19 pages, no figures, submitted to J. Phys.

Heats of Formation of Triplet Ethylene, Ethylidene, and Acetylene

Heats of formation of the lowest triplet state of ethylene and the ground triplet state of ethylidene have been predicted by high level electronic structure calculations. Total atomization energies obtained from coupled-cluster CCSD(T) energies extrapolated to the complete basis set limit using correlation consistent basis sets (CBS), plus additional corrections predict the following heats of formation in kcal/mol: Delta H0f(C2H4,3A1) = 80.1 at 0 K and 78.5 at 298 K, and Delta H0f(CH3CH,3A") = 86.8 at 0 K and 85.1 at 298 K, with an error of less than +-1.0 kcal/mol. The vertical and adiabatic singlet-triplet separation energies of ethylene were calculated as Delta ES-T,vert = 104.1 and Delta ES-T,adia = 65.8 kcal/mol. These results are in excellent agreement with recent quantum Monte Carlo (DMC) values of 103.5 +- 0.3 and 66.4 +- 0.3 kcal/mol. Both sets of computational values differ from the experimental estimate of 58 +- 3 kcal/mol for the adiabatic splitting. The computed singlet-triplet gap at 0 K for acetylene is Delta ES-T,adia(C2H2) = 90.5 kcal/mol, which is in notable disagreement with the experimental value of 82.6 kcal/mol. The heat of formation of the triplet is Delta H0f(C2H2,3B2) = 145.3 kcal/mol. There is a systematic underestimation of the singlet-triplet gaps in recent photodecomposition experiments by ~;;7 to 8 kcal/mol. For vinylidene, we predict Delta H0f(H2CC,1A1) = 98.8 kcal/mol at 298 K (exptl. 100.3 +- 4.0), Delta H0f(H2CC,3B2) = 146.2 at 298 K, and an energy gap Delta ES-T-adia(H2CC) = 47.7 kcal/mol

AB INITIO STUDY OF THE VIBRATIONAL SPECTRA OF NO3

The vibrational spectra and geometry of the NO, molecule is studied using ab initio SCF and CASSCF methods. For all levels of theory and basis set the highest symmetry found is Czv, Vibrational levels agree well with recent experimental results

SUSY parameter determination at the LHC using cross sections and kinematic edges

We study the determination of supersymmetric parameters at the LHC from a global fit including cross sections and edges of kinematic distributions. For illustration, we focus on a minimal supergravity scenario and discuss how well it can be constrained at the LHC operating at 7 and 14 TeV collision energy, respectively. We find that the inclusion of cross sections greatly improves the accuracy of the SUSY parameter determination, and allows to reliably extract model parameters even in the initial phase of LHC data taking with 7 TeV collision energy and 1/fb integrated luminosity. Moreover, cross section information may be essential to study more general scenarios, such as those with non-universal gaugino masses, and distinguish them from minimal, universal, models.Comment: 22 pages, 8 figure