We present studies using natural bond orbitals (NBOs) as a starting point for a localized electron correlation treatment, as these kind of localized orbitals lead to CCSD results which show significant transferability and exponential decay patterns in the T̂2 amplitudes. The NBO CCSD approach combines the advantages of both the HF CCSD formulation (less amplitudes, orthogonal orbitals) and the A...

The method of extracting effective atomic orbitals and effective minimal basis sets from molecular wave function characterizing the state of an atom in a molecule is developed in the framework of the "fuzzy" atoms. In all cases studied, there were as many effective orbitals that have considerable occupation numbers as orbitals in the classical minimal basis. That is considered to be of high con...

The theoretical description of electronic resonances is extended beyond calculations of energies and lifetimes. We present the formalism for calculating Dyson orbitals and transition dipole moments within the equation-of-motion coupled-cluster singles and doubles method for electron-attached states augmented by a complex absorbing potential (CAP-EOM-EA-CCSD). The capabilities of the new methodo...

We study the tree-tensor-network-state (TTNS) method with variable tensor orders for quantum chemistry. TTNS is a variational method to efficiently approximate complete active space (CAS) configuration interaction (CI) wave functions in a tensor product form. TTNS can be considered as a higher order generalization of the matrix product state (MPS) method. The MPS wave function is formulated as ...

By means of Many-Body Perturbation Theory (MBPT) it is shown that minimizing the energy expectation value of a closed-shell system in a certain order of the preturbation expansion by varying the partitioning of the Hamiltonian, leads to a zeroth-order function, which – as the order of perturbation increases and provided the expansion converges properly – approaches a determinant of Brueckner or...

Methods of efficiently optimizing the orbitals of generalized valence bond (GVB) wavefunctions are discussed and applied to LiH, BH, H3, H,O, C6Hs, and o,. The strong orthogonality and perfect pairing restrictions are tested for the X 1::!:+ state of LiH, the X 1::!:+, a 3II, and A 1II states of BH, and the H2+ D;::::: H + HD exchange reaction. The orbitals of H,O and C2H6 naturally localize in...

In this contribution, we study several monocarbonyl-metal complexes in order to unravel the contribution of relativistic effects to the metal-ligand bond length and complexation energy. Using scalar density functional theory (DFT) constrained space orbital variation (CSOV) energy decomposition analysis supplemented by all-electron four-component DFT computations, we describe the dependency of r...

The influence of short-range Coulomb correlations on the Mott transition in the single-band Hubbard model at half filling is studied within cellular dynamical mean-field theory for square and triangular lattices. Finitetemperature exact diagonalization is used to investigate correlations within two-, three-, and four-site clusters. Transforming the nonlocal self-energy from a site basis to a mo...