0000000000001539
AUTHOR
Stefano Evangelisti
Coupled-Cluster study of ‘no-pair’ bonding in the tetrahedral Cu4 cluster
Abstract Ab initio Coupled-Cluster calculations with single and double excitations and perturbative correction to the triple, CCSD(T), have been carried out for the high-spin electronic state, ( 5 A 2 ) , of the copper cluster Cu 4 in its tetrahedral arrangement. Like alkali metals clusters, tetrahedral Cu 4 presents a bound quintet state, i.e., a situation where all the valence electrons are unpaired. This rather exotic wavefunction, also known as no-pair bonding state, is examined in detail. The influence of the basis set is also analyzed, as well as the importance of the core correlation and the effect of the basis-set superposition errors.
Size-consistent self-consistent configuration interaction from a complete active space : Excited states
The self-consistent size consistent on a complete active space singly and doubly configuration interaction (SC)2CAS-SDCI method is applied to excited states. The (SC)2 correction is performed on a closed shell state, and the excited states are obtained by diagonalization of the dressed matrix. A theoretical justification of the transferability of the improvement concerning the dressing state to all roots of the matrix is presented. The method is tested by three tests on the spectrum of small molecules. sanchezm@uv.es ; nebot@uv.es
Full Configuration-Interaction Study on the Tetrahedral Li4 Cluster
International audience; The Li4 cluster low lying electronic states were studied. In particular we investigated the tetrahedral geometry at full CI and coupled cluster level, with basis sets of increasing quality. The 5A2 electronic state, characterized by having all the valence electrons unpaired, forming a quite stable no-pair bonding state, was studied in greater detail. In order to compare the energies we also studied the Li4 rhombus singlet ground state. The ability of coupled cluster with perturbative triples to correctly reproduce energy levels in a quasi-degenerate system was validated with respect to the full CI.
Ab initio study of the C60+Na system
Abstract In this work we present the first CAS-CI calculation of the potential-electronic curves for the lowest states of the C 60 +Na system using a set of local orbitals. These orbitals permit to select a small active space describing the ionic interaction between the C 60 and the Na atom. A binding energy of about 3 eV has been found, a value substantially larger than previous theoretical results.
Localized molecular orbitals for excited states of polyenals, polyendials, and polyenones
The work is focused on the generation of localized molecular orbitals for excited states. A recently developed a priori method based in a CAS-SCF–type algorithm is applied. The method generates directly localized orbitals and can be applied to multireference wavefunctions. A detailed description of the performance of the method as well as the locality of the MOs for the example of the singlet nπ* (CO) excited state is given. It is in general possible to obtain local orbitals for the doubly occupied and virtual valence orbitals. The partial delocalization of the π* (CO) orbital is discussed, as is the effect of the use of different CAS spaces. The systems under study are polyenals, polyendia…
Computing the position-spread tensor in the CAS-SCF formalism II: Spin partition
Abstract The Spin-Partitioned (SP) Total Position-Spread (TPS) tensor provides finer insights that supplement the information conveyed in the Spin-Summed (SS) TPS. The calculation of the SP-TPS has been implemented in the MOLPRO code for CAS-SCF wavefunctions allowing the study of electron (de) localization in relatively large molecular systems where the FCI treatment is rather unfeasible. An illustrative example considering one-dimensional Be wires is given as an application of the formalism.
Full configuration interaction calculation of Be3.
The full configuration interaction (FCI) study of the ground state of the neutral beryllium trimer has been performed using an atomic natural orbitals [3s2p1d] basis set. Both triangular and linear structures have been considered for the Be(3) cluster. The optimal geometry for the equilateral triangle has been calculated. The potential energy cut sections along the normal a(1)(') mode and one of the components of the e(') mode have then been studied. The FCI symmetric atomization potential of the linear cluster is also reported. It shows a secondary van der Waals minimum at a long bond distance. All singular points in the potential energy curves are characterized. Other properties, like dis…
Code Interoperability and Standard Data Formats in Quantum Chemistry and Quantum Dynamics: The Q5/Q5cost Data Model
Code interoperability and the search for domain-specific standard data formats represent critical issues in many areas of computational science. The advent of novel computing infrastructures such as computational grids and clouds make these issues even more urgent. The design and implementation of a common data format for quantum chemistry (QC) and quantum dynamics (QD) computer programs is discussed with reference to the research performed in the course of two Collaboration in Science and Technology Actions. The specific data models adopted, Q5Cost and D5Cost, are shown to work for a number of interoperating codes, regardless of the type and amount of information (small or large datasets) …
High-spin states in tetrahedral X4 clusters (X = H, Li, Na, K)
The high-spin electronic states for lithium, sodium, and potassium four-atom clusters were studied. In particular, we performed coupled cluster geometry optimization of the quintet state in tetrahedral geometry. The quintet state of these systems is characterized by having all the valence electron Unpaired, giving rise to the so-called no-pair bonding. Single-point full configuration interaction computations on the equilibrium geometries for the various Clusters are also presented. The analysis of the valence orbitals in a localized representation confirms the importance of the p atomic orbitals to explain this unusual type of bond. (C) 2009 Wiley Periodicals, Inc. Int J Quantum Chem 110: 8…
A Wigner molecule at extremely low densities: a numerically exact study
In this work we investigate Wigner localization at very low densities by means of the exact diagonalization of the Hamiltonian. This yields numerically exact results. In particular, we study a quasi-one-dimensional system of two electrons that are confined to a ring by three-dimensional gaussians placed along the ring perimeter. To characterize the Wigner localization we study several appropriate observables, namely the two-body reduced density matrix, the localization tensor and the particle-hole entropy. We show that the localization tensor is the most promising quantity to study Wigner localization since it accurately captures the transition from the delocalized to the localized state an…
The problem of interoperability: A common data format for quantum chemistry codes
A common format for quantum chemistry (QC), enhancing code interoperability and communication between different programs, has been designed and implemented. An XML-based format, QC-ML, is presented for representing quantities such as geometry, basis set, and so on, while an HDF5-based format is presented for the storage of large binary data files. Some preliminary applications that use the format have been implemented and are also described. This activity was carried out within the COST in Chemistry D23 project “MetaChem,” in the Working Group “A meta-laboratory for code integration in ab initio methods.” © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007
Full configuration interaction calculation of singlet excited states of Be3
The full configuration interaction (FCI) study of the singlets vertical spectrum of the neutral beryllium trimer has been performed using atomic natural orbitals [3s2p1d] basis set. The FCI triangular equilibrium structure of the ground state has been used to calculate the FCI vertical excitation energies up to 4.8 eV. The FCI vertical ionization potential for the same geometry and basis set amounts to 7.6292 eV. The FCI dipole and quadrupole transition moments from the ground state are reported as well. The FCI electric quadrupole moment of the X (3)A(1) (') ground state has been also calculated with the same basis set (Theta(zz)=-2.6461 a.u., Theta(xx)=Theta(yy)=-1/2Theta(zz)). Twelve of …
Multistate active spaces from local CAS-SCF molecular orbitals: the photodissociation of HFCO as an example.
A recently developed algorithm to generate localized molecular orbitals (LMO) is applied to the study of excited states along a photodissociation process. The LMOs allow for the selection of a consistent complete active space (CAS) for the simultaneous description of all the electronic states involved in a multistate process on the basis of simple chemical criteria. The local nature of the orbitals is used to label them in a unique way that does not depend on the molecular geometry. The selection of the electronic configurations of interest for the set of target states on only the basis of the dominant excitations required by the simplest configuration interaction (CI) descriptions for both…