الفهرس 
Chapter 1 IntroductionOnly 14 pages are availabe for public view
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Abstract
Quantum Monte Carlo (QMC) is one of the most promising methods for solving quantum
many body problems. However, in QMC methodology it is generally desirable to use the
simplest possible trial wavefunction since the most computationally intensive part of any
QMC simulation is evaluating the trial wavefunction and its derivatives. Therefore, using
highly correlated methods for wavefunction construction in QMC are limited to
comparatively small molecular systems. Instead, density functional theory DFT is an
efficient and popular way for constructing the QMC initial orbitals and at the same time
maintaining a reasonable scaling with system size. However, the mean obstacle of DFT is
that the true exchange-correlation density functional is unknown and the accuracy of
these exchange-correlation functionals depends on a particular application.
On the other hand, studying rare-earth containing systems is still a challenge for both
experimentalists and theoreticians. Despite, their technologically importance, many of
their properties remain unknown. In this regard, the objective of this work is to use the
QMC, and more specifically the DMC method, employing the suitable DFT trial function
to provide insights and predictions of some physical properties of species involving these
elements. To this aim, we assess the performance of a range of DFT, including pure,
hybrid, and long-range corrected functionals as well as Hartree-Fock function in order to
select the best starting place for our QMC trial function for systems containing rareearths.
This thesis is also relevant to calculate the potential energy curves PECs for the lowlying
states of lanthanum monoboride, monocarbide and monophosphide neutrals and
anions using the DMC method combined with three different trial functions. from the
fitted PECs, spectroscopic constants have been numerically determined and the ground
states have been assigned. For diatomic lanthanum boride and carbide, our results have
been compared with the only theoretical work exists in literature; however, predictions
have been provided for lanthanum phosphide which has not been explored yet. Moreover,
variations of the dipole moments with internuclear distances for the low-lying states of
the former neutral species have been studied and analyzed.
Having obtained the best DFT exchange-correlation functional convenient for
actinides, we also present in this thesis the first attempts to study the electronic structure of actinide monohydrides and monofluorides by means of the DMC method. Ground
state total energies, bond lengths, dissociation energies, and dipole moments have been
reported. Our results compare well with the few data available in literature, confirming
the ability of the DMC method to successfully describe the physical properties of these
molecules. Finally, an outlook on future topics of interest has been presented.