2,238 research outputs found
Rotational dynamics of molecular impurities solvated in 4He clusters. A computational study based on reptation quantum Monte Carlo
The thesis is organized as follows: in the first chapter we provide a short description
of the scenario which frames our work. Some significant experiments
are presented, together with the questions raised by them, the theoretical investigations
they stimulated, and the open issues. The second chapter describes the
reptation quantum Monte Carlo method and its theoretical foundations. Technical
aspects (the choice of the trial functions, the procedure to calculate the cluster
rotational energies) are discussed in third chapter. We also report some studies
on the reptation algorithm. In the fourth chapter we apply RQMC for the interpretation
of the infrared spectra of CO@HeN [5]. The fifth chapter addresses the
problem of the evolution of the rotational dinamics of He solvated rotors toward
the nanodroplet regime; two paradigmatic cases, OCS@HeN and HCN@HeN , are
studied. Our conclusions are drawn in the last chapter
QMCPACK: Advances in the development, efficiency, and application of auxiliary field and real-space variational and diffusion Quantum Monte Carlo
We review recent advances in the capabilities of the open source ab initio
Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for
greater efficiency and reproducibility. The auxiliary field QMC (AFQMC)
implementation has been greatly expanded to include k-point symmetries,
tensor-hypercontraction, and accelerated graphical processing unit (GPU)
support. These scaling and memory reductions greatly increase the number of
orbitals that can practically be included in AFQMC calculations, increasing
accuracy. Advances in real space methods include techniques for accurate
computation of band gaps and for systematically improving the nodal surface of
ground state wavefunctions. Results of these calculations can be used to
validate application of more approximate electronic structure methods including
GW and density functional based techniques. To provide an improved foundation
for these calculations we utilize a new set of correlation-consistent effective
core potentials (pseudopotentials) that are more accurate than previous sets;
these can also be applied in quantum-chemical and other many-body applications,
not only QMC. These advances increase the efficiency, accuracy, and range of
properties that can be studied in both molecules and materials with QMC and
QMCPACK
Ab initio molecular dynamics of water by quantum Monte Carlo
The chapter 2, we deal with the challenge b). It focuses on the variational
Monte Carlo (VMC) and the wavefunction optimization methods based on
VMC. The performance of different methods are displayed through the op-
timization of the Jastrow factor in our test case Beryllium dimer and the
efficiency is improving surprisingly during the evolution of these methods.
In chapter 3, we focus on the challenge a). It describes the wavefunc-
tion ansatz used by our simulation. In this thesis, we introduce the atomic
hybrid orbitals which significantly increase the compactness of our wavefunc-
tion without hurting accuracy. This chapter also explain how to optimize
the determinant in a way that the number of variational parameters scales
only linearly with the system size. This further helps the efficiency of the
wavefunction optimization.
In chapters 4 and 5, the issue c) is explained in detail. In chapter 4,
a second order Langevin dynamics (SLD) scheme is devised particularly for
QMC and this thesis improves this scheme by developing a better integration
method. Here, we also highlight the remarkable power of the force covari-
ance matrix which can be defined only in QMC and is capable of accelerating
the slow modes of a dynamics. In chapter 5, this SLD for QMC is validated
through intensive benchmarking on the calculation of the vibrational frequen-
cies of water and other small molecules. It is shown that many systematic
biases in our MD scheme and QMC evaluation can be controlled so that we
are confident to push forward this ab initio molecular dynamics for applica-
tions on large systems.
Finally in chapter 6, we perform the simulation of liquid water with all the
preparation done in the previous chapters. The results are encouraging since
we\u2019ve closed the discrepancy of the peak positions of RDFs between experi-
ments and ab initio simulations. The power of QMC is also demonstrated by
the fact that the shapes of our RDFs are much less structured than previous
DFT-based ab initio simulations even if the two water molecule interaction is
dealt with the same level of accuracy as the DFT/BLYP calculation. In this
chapter, we have also studied the features of hydrogen bonds in our simulation
of liquid water. All our results indicate that it is important to consider the
quantum nature of the ions for a faithful description of liquid water. This will
be left for future studies, possible in principle even within the QMC approach
Simulation studies for surfaces and materials strength
A realistic potential energy function comprising angle dependent terms was employed to describe the potential surface of the N+O2 system. The potential energy parameters were obtained from high level ab-initio results using a nonlinear fitting procedure. It was shown that the potential function is able to reproduce a large number of points on the potential surface with a small rms deviation. A literature survey was conducted to analyze exclusively the status of current small cluster research. This survey turned out to be quite useful in understanding and finding out the existing relationship between theoretical as well as experimental investigative techniques employed by different researchers. Additionally, the importance of the role played by computer simulation in small cluster research, was documented
Quantum Dissipative Dynamics and Decoherence of Dimers on Helium Droplets
In this thesis, quantum dynamical simulations are performed in order to describe the vibrational motion of diatomic molecules in a highly quantum environment, so-called helium droplets. We aim to reproduce and explain experimental findings which were obtained from dimers on helium droplets. Nanometer-sized helium droplets contain several thousands of 4-He atoms. They serve as a host for embedded atoms or molecules and provide an ultracold “refrigerator” for them. Spectroscopy of molecules in or on these droplets reveals information on both the molecule and the helium environment. The droplets are known to be in the superfluid He II phase. Superfluidity in nanoscale systems is a steadily growing field of research.
Spectra obtained from full quantum simulations for the unperturbed dimer show deviations from measurements with dimers on helium droplets. These deviations result from the influence of the helium environment on the dimer dynamics. In this work, a well-established quantum optical master equation is used in order to describe the dimer dynamics effectively. The master equation allows to describe damping fully quantum mechanically. By employing that equation in the quantum dynamical simulation, one can study the role of dissipation and decoherence in dimers on helium droplets.
The effective description allows to explain experiments with Rb-2 dimers on helium droplets. Here, we identify vibrational damping and associated decoherence as the main explanation for the experimental results. The relation between decoherence and dissipation in Morse-like systems at zero temperature is studied in more detail.
The dissipative model is also used to investigate experiments with K-2 dimers on helium droplets. However, by comparing numerical simulations with experimental data, one finds that further mechanisms are active. Here, a good agreement is obtained through accounting for rapid desorption of dimers. We find that decoherence occurs in the electronic manifold of the molecule. Finally, we are able to examine whether superfluidity of the host does play a role in these experiments.In dieser Dissertation werden quantendynamische Simulationen durchgeführt, um die Schwingungsbewegung zweiatomiger Moleküle in einer hochgradig quantenmechanischen Umgebung, sogenannten Heliumtröpfchen, zu beschreiben. Unser Ziel ist es, experimentelle Befunde zu reproduzieren und zu erklären, die von Dimeren auf Heliumtröpfchen erhalten wurden.
Nanometergroße Heliumtröpfchen enthalten einige tausend 4-He Atome. Sie dienen als Wirt für eingebettete Atome oder Moleküle und stellen für dieseeinen ultrakalten „Kühlschrank“ bereit. Durch Spektroskopie mit Molekülen in oder auf diesen Tröpfchen erhält man Informationen sowohl über das Molekül selbst als auch über die Heliumumgebung. Man weiß, dass sich die Tröpfchen in der suprafluiden He II Phase befinden. Suprafluidität in Nanosystemen ist ein stetig wachsendes Forschungsgebiet.
Spektren, die für das ungestörte Dimer durch voll quantenmechanische Simulationen erhalten werden, weichen von Messungen mit Dimeren auf Heliumtröpfchen ab. Diese Abweichungen lassen sich auf den Einfluss der Heliumumgebung auf die Dynamik des Dimers zurückführen. In dieser Arbeit wird eine etablierte quantenoptische Mastergleichung verwendet, um die Dynamik des Dimers effektiv zu beschreiben. Die Mastergleichung erlaubt es, Dämpfung voll quantenmechanisch zu beschreiben. Durch Verwendung dieser Gleichung in der Quantendynamik-Simulation lässt sich die Rolle von Dissipation und Dekohärenz in Dimeren auf Heliumtröpfchen untersuchen.
Die effektive Beschreibung erlaubt es, Experimente mit Rb-2 Dimeren zu erklären. In diesen Untersuchungen wird Dissipation und die damit verbundene Dekohärenz im Schwingungsfreiheitsgrad als maßgebliche Erklärung für die experimentellen Resultate identifiziert. Die Beziehung zwischen Dekohärenz und Dissipation in Morse-artigen Systemen bei Temperatur Null wird genauer untersucht.
Das Dissipationsmodell wird auch verwendet, um Experimente mit K-2 Dimeren auf Heliumtröpfchen zu untersuchen. Wie sich beim Vergleich von numerischen Simulationen mit experimentellen Daten allerdings herausstellt, treten weitere Mechanismen auf. Eine gute Übereinstimmung wird erzielt, wenn man eine schnelle Desorption der Dimere berücksichtigt. Wir stellen fest, dass ein Dekohärenzprozess im elektronischen Freiheitsgrad des Moleküls auftritt. Schlussendlich sind wir in der Lage herauszufinden, ob Suprafluidität des Wirts in diesen Experimenten eine Rolle spielt
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