Analytic one-electron properties at the 4-component relativistic coupled cluster level with inclusion of spin-orbit coupling

International audienceArticles you may be interested in Description of spin-orbit coupling in excited states with two-component methods based on approximate coupled-cluster theory An ab initio two-component relativistic method including spin-orbit coupling using the regular approximation We present a formulation and implementation of the calculation of (orbital-unrelaxed) expectation values at the 4-component relativistic coupled cluster level with spin-orbit coupling included from the start. The Lagrangian-based analytical energy derivative technique constitutes the basic theoretical framework of this work. The key algorithms for single reference relativistic coupled cluster have been implemented using routines for general tensor contractions of up to rank-2 tensors in which the direct product decomposition scheme is employed to benefit from double group symmetry. As a sample application, we study the electric field gradient at the bismuth nucleus in the BiX (X = N, P) series of molecules, where the effect of spin-orbit coupling is substantial. Our results clearly indicate that the current reference value for the nuclear quadrupole moment of 209 Bi needs revision. We also have applied our method to the calculation of the parity violating energy shift of chiral molecules. The latter property is strictly zero in the absence of spin-orbit coupling. For the H 2 X 2 (X = O,S,Se,Te) series of molecules the effect of correlation is found to be quite small. Published by AIP Publishing. [http://dx

Exploring Coupled Cluster Green's function as a method for treating system and environment in Green's function embedding methods

Within the self-energy embedding theory (SEET) framework, we study coupled cluster Green's function (GFCC) method in two different contexts: as a method to treat either the system or environment present in the embedding construction. Our study reveals that when GFCC is used to treat the environment we do not see improvement in total energies in comparison to the coupled cluster method itself. To rationalize this puzzling result, we analyze the performance of GFCC as an impurity solver with a series of transition metal oxides. These studies shed light on strength and weaknesses of such a solver and demonstrate that such a solver gives very accurate results when the size of the impurity is small. We investigate if it is possible to achieve a systematic accuracy of the embedding solution when we increase the size of the impurity problem. We found that in such a case, the performance of the solver worsens, both in terms of finding the ground state solution of the impurity problem as well as the self-energies produced. We concluded that increasing the rank of GFCC solver is necessary to be able to enlarge impurity problems and achieve a reliable accuracy. We also have shown that natural orbitals from weakly correlated perturbative methods are better suited than symmetrized atomic orbitals (SAO) when the total energy of the system is the target quantity

Equation-of-motion coupled-cluster theory based on the 4-component Dirac-Coulomb(-Gaunt) Hamiltonian:Energies for single electron detachment, attachment, and electronically excited states

<p>This entry contains the figures included in the paper titled "Equation-of-Motion Coupled-Cluster Theory based on the 4-component Dirac--Coulomb(--Gaunt) Hamiltonian. Energies for single electron detachment, attachment and electronically excited states", by Avijit Shee, Trond Saue, Lucas Visscher and Andre Severo Pereira Gomes.</p> <p>It accompanies the dataset found at the DOI: 10.5281/zenodo.1320320</p> <p>There are three figures that use the (original) png files included in <a href="https://zenodo.org/api/files/7bda2e2b-ac69-41aa-a21e-821e88bfb973/original-figures.tar.bz2">original-figures.tar.bz2 </a>:</p> <p>figure 1: Potential energy curves of the spin-orbit split X²Π and A²Π states of the XO molecules, obtained with EOM-IP and the ²DCG^M Hamiltonian.</p> <p>figure 2: Internuclear distances (in Angstrom), harmonic vibrational frequencies (in cm⁻¹) and the vertical Ω = 3/2 − 1/2 energy difference (in eV) for the X²Π and A²Π states of the XO molecules, obtained with EOM-IP and the ²DCG^M Hamiltonian.</p> <p>figure 3: SO-ZORA/QZ4P/Hartree-Fock (ADF) spinor magnetization plots (isosurfaces at 0.03 a.u.) and energies (in Eh) for the valence spinors of the XO⁻ species (from left to right: X = Cl, Br, I, At, Ts).</p

Triple excitations in Green's function coupled cluster solver for studies of strongly correlated systems in the framework of self-energy embedding theory

Embedding theories became important approaches used for accurate calculations of both molecules and solids. In these theories, a small chosen subset of orbitals is treated with an accurate method, called an impurity solver, capable of describing higher correlation effects. Ideally, such a chosen fragment should contain multiple orbitals responsible for the chemical and physical behavior of the compound. Handing a large number of chosen orbitals presents a very significant challenge for the current generation of solvers used in the physics and chemistry community. Here, we develop a Green's function coupled cluster singles doubles and triples (GFCCSDT) solver that can be used for a quantitative description in both molecules and solids. This solver allows us to treat orbital spaces that are inaccessible to other accurate solvers. At the same time, GFCCSDT maintains high accuracy of the resulting self-energy. Moreover, in conjunction with the GFCCSD solver, it allows us to test the systematic convergence of computational studies. Developing the CC family of solvers paves the road to fully systematic Green's function embedding calculations in solids. In this paper, we focus on the investigation of GFCCSDT self-energies for a strongly correlated problem of SrMnO$_3$ solid. Subsequently, we apply this solver to solid MnO showing that an approximate variant of GFCCSDT is capable of yielding a high accuracy orbital resolved spectral function

Seasonal evolution of oceanic upper layer processes in the northern Bay of Bengal following a single Argo float

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 46(10), (2019): 5369-5377, doi: 10.1029/2019GL082078.Seasonal evolution of the barrier layer (BL) and temperature inversion in the northern Bay of Bengal and their role on the mixed layer temperature (MLT) is examined using observations from a single Argo during December 2013 to July 2017. During fall, low salinity at surface generates BL in this region. It thickens to almost 80 m in winter enhanced by deepening of isothermal layer depth due to remote forcing. During winter, surface cooling lowers near‐surface temperature, and thus, the subsurface BL experiences a significant temperature inversion (~2.5 °C). This temperature inversion diffuses to distribute heat within ML and surface heating begins deep penetration of shortwave radiation through ML during spring. Hence, the ML becomes thermally well stratified, resulting in the warmest MLT. The Monin‐Obukhov length attains its highest value during summer indicating wind dominance in the ML. During spring and fall, upper ocean gains heat allowing buoyancy to dominate over wind mixing.A. S. and S. S. thank financial support from Space Application Centre (SAC), Indian Space Research Organization (ISRO), Government of India (Grant: SAC/EPSA/4.19/2016). This study was also supported by the first phase of Ministry of Earth Sciences (MoES), Government of India grant to establish a Bay of Bengal Coastal Observatory (BOBCO) at IITBBS (Grant: RP088). Authors acknowledged NCPOR Contribution number J ‐ 03/2019‐20 for this work. The authors are grateful to the reviewers and the Editor for constructive suggestions. The figures are generated using Matlab. The data source and availability are given in the Text S1.2019-10-2

Relativistic EOM-CCSD for Core-Excited and Core-Ionized State Energies Based on the Four-Component Dirac–Coulomb(−Gaunt) Hamiltonian

We report an implementation of the core–valence separation approach to the four-component relativistic Hamiltonian-based equation-of-motion coupled-cluster with singles and doubles theory (CVS-EOM-CCSD) for the calculation of relativistic core-ionization potentials and core-excitation energies. With this implementation, which is capable of exploiting double group symmetry, we investigate the effects of the different CVS-EOM-CCSD variants and the use of different Hamiltonians based on the exact two-component (X2C) framework on the energies of different core-ionized and -excited states in halogen- (CH3I, HX, and X–, X = Cl–At) and xenon-containing (Xe, XeF2) species. Our results show that the X2C molecular mean-field approach [Sikkema, J.; J. Chem. Phys. 2009, 131, 124116], based on four-component Dirac–Coulomb mean-field calculations (2DCM), is capable of providing core excitations and ionization energies that are nearly indistinguishable from the reference four-component energies for up to and including fifth-row elements. We observe that two-electron integrals over the small-component basis sets lead to non-negligible contributions to core binding energies for the K and L edges for atoms such as iodine or astatine and that the approach based on Dirac–Coulomb–Gaunt mean-field calculations (2DCGM) are significantly more accurate than X2C calculations for which screened two-electron spin–orbit interactions are included via atomic mean-field integrals

The DIRAC code for relativistic molecular calculations

DIRAC is a freely distributed general-purpose program system for one-, two-, and four-component relativistic molecular calculations at the level of Hartree?Fock, Kohn?Sham (including range-separated theory), multiconfigurational self-consistent-field, multireference configuration interaction, electron propagator, and various flavors of coupled cluster theory. At the self-consistent-field level, a highly original scheme, based on quaternion algebra, is implemented for the treatment of both spatial and time reversal symmetry. DIRAC features a very general module for the calculation of molecular properties that to a large extent may be defined by the user and further analyzed through a powerful visualization module. It allows for the inclusion of environmental effects through three different classes of increasingly sophisticated embedding approaches: the implicit solvation polarizable continuum model, the explicit polarizable embedding model, and the frozen density embedding model.Fil: Saue, Trond. Université Paul Sabatier; Francia. Centre National de la Recherche Scientifique; FranciaFil: Bast, Radovan. Uit The Arctic University Of Norway; NoruegaFil: Gomes, André Severo Pereira. University Of Lille.; Francia. Centre National de la Recherche Scientifique; FranciaFil: Jensen, Hans Jorgen Aa.. University of Southern Denmark; DinamarcaFil: Visscher, Lucas. Vrije Universiteit Amsterdam; Países BajosFil: Aucar, Ignacio Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Nordeste. Instituto de Modelado e Innovación Tecnológica. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas Naturales y Agrimensura. Instituto de Modelado e Innovación Tecnológica; Argentina. Universidad Nacional del Nordeste. Facultad de Ciencias Exactas y Naturales y Agrimensura. Departamento de Física; ArgentinaFil: Di Remigio, Roberto. Uit The Arctic University of Norway; NoruegaFil: Dyall, Kenneth G.. Dirac Solutions; Estados UnidosFil: Eliav, Ephraim. Universitat Tel Aviv.; IsraelFil: Fasshauer, Elke. Aarhus University. Department of Bioscience; DinamarcaFil: Fleig, Timo. Université Paul Sabatier; Francia. Centre National de la Recherche Scientifique; FranciaFil: Halbert, Loïc. Centre National de la Recherche Scientifique; Francia. University Of Lille.; FranciaFil: Hedegård, Erik Donovan. Lund University; SueciaFil: Helmich-Paris, Benjamin. Max-planck-institut Für Kohlenforschung; AlemaniaFil: Ilias, Miroslav. Matej Bel University; EslovaquiaFil: Jacob, Christoph R.. Technische Universität Braunschweig; AlemaniaFil: Knecht, Stefan. Eth Zürich, Laboratorium Für Physikalische Chemie; SuizaFil: Laerdahl, Jon K.. Oslo University Hospital; NoruegaFil: Vidal, Marta L.. Department Of Chemistry; DinamarcaFil: Nayak, Malaya K.. Bhabha Atomic Research Centre; IndiaFil: Olejniczak, Malgorzata. University Of Warsaw; PoloniaFil: Olsen, Jógvan Magnus Haugaard. Uit The Arctic University Of Norway; NoruegaFil: Pernpointner, Markus. Kybeidos Gmbh; AlemaniaFil: Senjean, Bruno. Universiteit Leiden; Países BajosFil: Shee, Avijit. Department Of Chemistry; Estados UnidosFil: Sunaga, Ayaki. Tokyo Metropolitan University; JapónFil: van Stralen, Joost N. P.. Vrije Universiteit Amsterdam; Países Bajo

Relativistic coupled cluster theory - in molecular properties and in electronic structure

L'importance des effets relativistes dans la chimie a été reconnu depuis les années 1980. Par exemple, sans la relativité (a) l'or aurait la même couleur que l'argent (b) le mercure ne serait pas liquide à la température ambiante et (c) nos voitures ne démarrent pas avec une batterie de plomb. Pour une description théorique de la structure et la réactivité des éléments lourds, la relativité est un ingrédient essentiel. Le hamiltonien pour les calculs moléculaires relativistes à 4 composantes est construit en remplaçant la partie mono-électronique de l'hamiltonien électronique non-relativiste par le hamiltonien de Dirac. La partie bi-électronique est approchée par le terme de r Coulomb comme dans le cas non relativiste, ce qui donnel'hamiltonien de Dirac-Coulomb (DC). Pour réduire le coût de calcul, on peut utiliser des hamiltoniens relativistes à 2 composantes. Parmi eux, l'hamiltonien exact à 2 composantes (X2C) est le plus précise. La corrélation électronique est, cependant, une contribution très importante pour obtenir une description théorique à la fois qualitative et quantitative des spectroscopies moléculaires, réactions, etc. Dans cette thèse, nous avons étudié l'interaction entre la relativité et de la corrélation. à la fois par des développements méthodologiques et par des applications moléculaires. Dans la première partie de la thèse, nous avons calculé les constantes spectroscopiques dimères des gaz rares lourds. La liaison faible de ces dimères ne peut être décrit que par l'inclusion de la corrélation électronique. Les dimères des gaz rares les plus lourds, le radon et l'eka-radon, nécessite de plus un traitement adéquat de la relativité. Nos calculs sont basés sur l'hamiltonien X2Cmmf, à la fois avec des méthodes de corrélation basés sur une fonction d'onde et séparation de porte (srDFT). La deuxième partie de cette thèse concerne la simulation de la spectroscopie des rayons X, où l'on sonde la région du cœur d'une molécule, ou la relativité joue un rôle très important. Nous avons étudié la spectroscopie L-edge de la série isoélectronique: UO22 +, UNO+, et UN2, où le couplage spin-orbite joue un rôle majeur. Au niveau des méthodes, nous avons considéré MP2 à couches ouvertes et la théorie de la fonctionnelle de la densité dépendante de temps (TDDFT). Dans un autre étude, nous avons simulé la spectroscopie K-edge de la série H2X (X = O, S, Se, Te) et XH3 (X = N, P, As) ainsi que les molécules N2 et N2O2. Pour ces systèmes, l'interaction spin-orbite est moins important. Par conséquent, nous avons utilisé un hamiltonien DC sans spin (SF). Certains des systèmes pris en compte dans ce travail sont de caractère multi-référentielles ; nous avons utilisé la methode Coupled Cluster Multi-référentielle de type State Universal et adapté au groupe unitaire (UGA-SUMRCC) comme une méthode de corrélation. Dans la troisième et partie principale de la thèse, l'attention est de nouveau sur la relativité et de la corrélation, mais pour le calcul des propriétés électriques et magnétiques moléculaires. Nous avons développé et mis en œuvre un module pour le calcul des valeurs moyennes au niveau relativiste à 4-composantes coupled cluster monoréferentiel. Les propriétés qui sondent la densité électronique près de noyaux (lourds), telles que la résonance paramagnétique électronique (RPE), les paramètres des gradients de champ électrique et la non-conservation de la parité (NCP) des molécules chirales ,sont parfaitement adaptés pour l'application de cette méthode. Pour l'instant, nous avons étudié que la NCP. Ce module dans le logiciel DIRAC pour les calculs moléculaires relativistes fournit un cadre propice pour la mise en œuvre de méthodes de CC relativistes employant la symétrie de groupes doubles et de permutation de manière très efficace. En perspective, nous ciblons la mise en œuvre de la réponse linéaire CC pour le calcul des énergies d'excitation et propriétés moléculaires de second ordre tels que les paramètres de RMN.The importance of relativistic effects in chemistry has been recognized since the 1980s. Without relativity (a) gold would have the same colour as silver (b) mercury would not be liquid at room temperature (c) our cars would not start (lead-battery). For a theoretical description of the structure and reactivity of heavy-elements, relativity is considered as an essential ingredient. The Hamiltonian for the 4-component relativistic molecular calculations is constructed by replacing the one-electronic part of the non-relativistic molecular Hamiltonian by the Dirac Hamiltonian. The two-electronic part of the Hamiltonian is approximated by the Coulombic repulsion term as in the non-relativistic case. The resulting Hamiltonian is called the Dirac-Coulomb (DC) Hamiltonian. For chemical applications there exist a class of relativistic Hamiltonians, where one-electronic part of the DC Hamiltonian is transformed to a 2-component one. Among them the eXcact 2-component (X2C) Hamiltonian is the most accurate one. Electron correlation, however, is a very important contribution to achieve a both qualitative and quantitative correct description of molecular spectroscopies, reactions etc. It is, therefore, essential to study the interplay between relativity and correlation. In this thesis, we have studied this interplay both in terms methodological developments and molecular applications. In the first part of the thesis we have studied the spectroscopic constants of the heavy rare gas dimers. The weak bonding of those dimers can only be described by the inclusion of electron correlation. The heavier analogues in the rare gas series i.e, Radon and eka-Radon, in addition require adequate treatment of relativity. Our calculations are based on the eXact 2-Component molecular-mean field (X2Cmmf) Hamiltonian both with wave function methods and range-separated DFT methods. The second part of this thesis simulates X-ray spectroscopy, where one probes the core region of a molecule. In the core region relativity plays a very significant role. Removal and excitation of electrons from that region involve various processes, which are beyond a mean-field description. We have studied L-edge spectroscopy of the isoelectronic series: UO22+, UNO+, and UN2, where spin-orbit coupling plays a major role. For the theory we have considered single reference open-shell MP2 and Time Dependent Density functional Theory (TDDFT). In another work, we have studied K-edge spectroscopy of the H2X (X= O, S, Se, Te) and XH3 (X= N, P, As) series as well as N2, N2O2 molecules. For this study spin-orbit coupling is less important, therefore, we have treated them with the Spin-Free (SF) DC Hamiltonian. Some of the systems considered in this work are Multi-Reference in nature; we have used Unitary Group Adapted (UGA) State Universal Multi-reference Coupled Cluster (UGA-SUMRCC) theory as a correlation method. In the third and major part of the thesis, the thrust is again on relativity and correlation, but for the calculation of molecular electric and magnetic properties. We have developed and implemented a module for the calculation of expectation values at the 4-component Relativistic Single Reference Coupled Cluster level. Properties that probe the electron density near (heavy) nuclei, such as Electron Paramagnetic Resonance (EPR) parameters, electric field gradients and parity non-conservation (PNC) in chiral molecules are ideally suited for the application of this method. However, we have only studied PNC so far. This module in the DIRAC software for relativstic molecular calculations provides a convenient framework for the implementation of relativistic CC methods employing double group and permutation symmetry very efficiently. In the near future we therefore target the implementation of Linear Response CC for the calculation of excitation energies and second-order molecular properties such as NMR parameters

La théorie coupled cluster relativiste - pour le calcul de la structure électronique et des propriétés moléculaires

L'importance des effets relativistes dans la chimie a été reconnu depuis les années 1980. Par exemple, sans la relativité (a) l'or aurait la même couleur que l'argent (b) le mercure ne serait pas liquide à la température ambiante et (c) nos voitures ne démarrent pas avec une batterie de plomb. Pour une description théorique de la structure et la réactivité des éléments lourds, la relativité est un ingrédient essentiel. Le hamiltonien pour les calculs moléculaires relativistes à 4 composantes est construit en remplaçant la partie mono-électronique de l'hamiltonien électronique non-relativiste par le hamiltonien de Dirac. La partie bi-électronique est approchée par le terme de r Coulomb comme dans le cas non relativiste, ce qui donnel'hamiltonien de Dirac-Coulomb (DC). Pour réduire le coût de calcul, on peut utiliser des hamiltoniens relativistes à 2 composantes. Parmi eux, l'hamiltonien exact à 2 composantes (X2C) est le plus précise. La corrélation électronique est, cependant, une contribution très importante pour obtenir une description théorique à la fois qualitative et quantitative des spectroscopies moléculaires, réactions, etc. Dans cette thèse, nous avons étudié l'interaction entre la relativité et de la corrélation. à la fois par des développements méthodologiques et par des applications moléculaires. Dans la première partie de la thèse, nous avons calculé les constantes spectroscopiques dimères des gaz rares lourds. La liaison faible de ces dimères ne peut être décrit que par l'inclusion de la corrélation électronique. Les dimères des gaz rares les plus lourds, le radon et l'eka-radon, nécessite de plus un traitement adéquat de la relativité. Nos calculs sont basés sur l'hamiltonien X2Cmmf, à la fois avec des méthodes de corrélation basés sur une fonction d'onde et séparation de porte (srDFT). La deuxième partie de cette thèse concerne la simulation de la spectroscopie des rayons X, où l'on sonde la région du cœur d'une molécule, ou la relativité joue un rôle très important. Nous avons étudié la spectroscopie L-edge de la série isoélectronique: UO22 +, UNO+, et UN2, où le couplage spin-orbite joue un rôle majeur. Au niveau des méthodes, nous avons considéré MP2 à couches ouvertes et la théorie de la fonctionnelle de la densité dépendante de temps (TDDFT). Dans un autre étude, nous avons simulé la spectroscopie K-edge de la série H2X (X = O, S, Se, Te) et XH3 (X = N, P, As) ainsi que les molécules N2 et N2O2. Pour ces systèmes, l'interaction spin-orbite est moins important. Par conséquent, nous avons utilisé un hamiltonien DC sans spin (SF). Certains des systèmes pris en compte dans ce travail sont de caractère multi-référentielles ; nous avons utilisé la methode Coupled Cluster Multi-référentielle de type State Universal et adapté au groupe unitaire (UGA-SUMRCC) comme une méthode de corrélation. Dans la troisième et partie principale de la thèse, l'attention est de nouveau sur la relativité et de la corrélation, mais pour le calcul des propriétés électriques et magnétiques moléculaires. Nous avons développé et mis en œuvre un module pour le calcul des valeurs moyennes au niveau relativiste à 4-composantes coupled cluster monoréferentiel. Les propriétés qui sondent la densité électronique près de noyaux (lourds), telles que la résonance paramagnétique électronique (RPE), les paramètres des gradients de champ électrique et la non-conservation de la parité (NCP) des molécules chirales ,sont parfaitement adaptés pour l'application de cette méthode. Pour l'instant, nous avons étudié que la NCP. Ce module dans le logiciel DIRAC pour les calculs moléculaires relativistes fournit un cadre propice pour la mise en œuvre de méthodes de CC relativistes employant la symétrie de groupes doubles et de permutation de manière très efficace. En perspective, nous ciblons la mise en œuvre de la réponse linéaire CC pour le calcul des énergies d'excitation et propriétés moléculaires de second ordre tels que les paramètres de RMN.The importance of relativistic effects in chemistry has been recognized since the 1980s. Without relativity (a) gold would have the same colour as silver (b) mercury would not be liquid at room temperature (c) our cars would not start (lead-battery). For a theoretical description of the structure and reactivity of heavy-elements, relativity is considered as an essential ingredient. The Hamiltonian for the 4-component relativistic molecular calculations is constructed by replacing the one-electronic part of the non-relativistic molecular Hamiltonian by the Dirac Hamiltonian. The two-electronic part of the Hamiltonian is approximated by the Coulombic repulsion term as in the non-relativistic case. The resulting Hamiltonian is called the Dirac-Coulomb (DC) Hamiltonian. For chemical applications there exist a class of relativistic Hamiltonians, where one-electronic part of the DC Hamiltonian is transformed to a 2-component one. Among them the eXcact 2-component (X2C) Hamiltonian is the most accurate one. Electron correlation, however, is a very important contribution to achieve a both qualitative and quantitative correct description of molecular spectroscopies, reactions etc. It is, therefore, essential to study the interplay between relativity and correlation. In this thesis, we have studied this interplay both in terms methodological developments and molecular applications. In the first part of the thesis we have studied the spectroscopic constants of the heavy rare gas dimers. The weak bonding of those dimers can only be described by the inclusion of electron correlation. The heavier analogues in the rare gas series i.e, Radon and eka-Radon, in addition require adequate treatment of relativity. Our calculations are based on the eXact 2-Component molecular-mean field (X2Cmmf) Hamiltonian both with wave function methods and range-separated DFT methods. The second part of this thesis simulates X-ray spectroscopy, where one probes the core region of a molecule. In the core region relativity plays a very significant role. Removal and excitation of electrons from that region involve various processes, which are beyond a mean-field description. We have studied L-edge spectroscopy of the isoelectronic series: UO22+, UNO+, and UN2, where spin-orbit coupling plays a major role. For the theory we have considered single reference open-shell MP2 and Time Dependent Density functional Theory (TDDFT). In another work, we have studied K-edge spectroscopy of the H2X (X= O, S, Se, Te) and XH3 (X= N, P, As) series as well as N2, N2O2 molecules. For this study spin-orbit coupling is less important, therefore, we have treated them with the Spin-Free (SF) DC Hamiltonian. Some of the systems considered in this work are Multi-Reference in nature; we have used Unitary Group Adapted (UGA) State Universal Multi-reference Coupled Cluster (UGA-SUMRCC) theory as a correlation method. In the third and major part of the thesis, the thrust is again on relativity and correlation, but for the calculation of molecular electric and magnetic properties. We have developed and implemented a module for the calculation of expectation values at the 4-component Relativistic Single Reference Coupled Cluster level. Properties that probe the electron density near (heavy) nuclei, such as Electron Paramagnetic Resonance (EPR) parameters, electric field gradients and parity non-conservation (PNC) in chiral molecules are ideally suited for the application of this method. However, we have only studied PNC so far. This module in the DIRAC software for relativstic molecular calculations provides a convenient framework for the implementation of relativistic CC methods employing double group and permutation symmetry very efficiently. In the near future we therefore target the implementation of Linear Response CC for the calculation of excitation energies and second-order molecular properties such as NMR parameters