Variance-based wave function optimization within the unrestricted doubly occupied configuration interaction framework: A half-projection treatment.

The energy-variance-based optimization procedures have proven to be useful tools to describe N-electron spectra. However, the resulting wave functions usually present spin-contaminant contributions. The goal of this work is to reduce the spin contamination of the results arising from the unrestricted doubly occupied configuration interaction method in its energy variance minimization version [Alcoba et al.

Generalized Spin in the Variance-Based Wave Function Optimization Method within the Doubly Occupied Configuration Interaction Framework.

In this work, we implement a generalized spin formulation of the doubly occupied configuration interaction methodology using the energy variance of the -electron Hamiltonian. We perform the optimization of the -electron wave functions and calculate their corresponding energies, using a unified variational treatment for ground and excited states based on the energy variance, which allows us to describe the entire energy spectra on an equal footing. We analyze the effects produced by the breakdown of the and symmetries in the spectra of model hydrogenic clusters in terms of energies and spin-related quantities, arising from the restricted, unrestricted, and generalized spin methods.

DOCSIC: A Mean-Field Method for Orbital-by-Orbital Self-Interaction Correction.

We introduce a new method to remove the one-electron self-interaction error in approximate density functional calculations on an orbital-by-orbital basis, as originally proposed by Perdew and Zunger [ , , 5048]. This method is motivated by a recent proposal by Pederson et al. [ , , 121103] to remove self-interaction that employs orbitals derived from the real-space density matrix, known as FLOSIC (Fermi Löwdin orbitals self-interaction correction).

A variance-based optimization for determining ground and excited N-electron wave functions within the doubly occupied configuration interaction scheme.

This work describes optimizations of N-electron system wave functions by means of the simulated annealing technique within the doubly occupied configuration interaction framework. Using that technique, we minimize the energy variance of a Hamiltonian, providing determinations of wave functions corresponding to ground or excited states in an identical manner. The procedure that allows us to determine electronic spectra can be performed using treatments of restricted or unrestricted types.

Determination of electronic excitation energies within the doubly occupied configuration interaction space by means of the Hermitian operator method.

In this work, we formulate the equations of motion corresponding to the Hermitian operator method in the framework of the doubly occupied configuration interaction space. The resulting algorithms turn out to be considerably simpler than the equations provided by that method in more conventional spaces, enabling the determination of excitation energies in N-electron systems under an affordable polynomial computational cost. The implementation of this technique only requires to know the elements of low-order reduced density matrices of an N-electron reference state, which can be obtained from any approximate method.

Generalized spin σ-SCF method.

We introduce a generalization of the σ-SCF method to approximate noncollinear spin ground and excited single-reference electronic states by minimizing the Hamiltonian variance. The new method is based on the σ-SCF method, originally proposed by Ye et al. [J.

Two Shared Icosahedral Metallacarboranes through Iron: A Joint Experimental and Theoretical Refinement of Mössbauer Spectrum in [Fe(1,2-CBH)]Cs.

Mössbauer and X-ray photoelectron spectroscopies (XPS) are complemented with high-level quantum-chemical computations in the study of the geometric and electronic structure of the paramagnetic salt of the metallacarborane sandwich complex [Fe(1,2-CBH)]Cs = FeSanCs. Experimental Fe isomer shifts and quadrupole splitting parameters are compared with the theoretical prediction, with good agreement. The appearance of two sets of Cs(3d) doublets in the XPS spectrum, separated by 2 eV, indicates that Cs has two different chemical environments due to ease of the Cs cation moving around the sandwich complex with low-energy barriers, as confirmed by quantum-chemical computations.

Influence of the N-representability conditions on the variational determination of two-electron reduced density matrices for ground and excited N-electron states in the doubly occupied configuration interaction space.

In this work, we analyze the effectiveness of different sets of well-known necessary N-representability conditions that are imposed on the variational determination of two-electron reduced density matrices corresponding to the ground and excited states of N-electron Hamiltonians within doubly occupied configuration interaction subspaces. The two-electron reduced density matrix elements are determined so that they are suitably related to those of higher-order reduced density matrices and other associated matrices, up to a given order, constituting positive semidefinite matrices. These conditions up to two-, three-, and four-order have been sequentially imposed on the variational calculation of the expectation values of the dispersion operator corresponding to several Hamiltonian models, obtaining the electronic spectra.

Variational determination of the two-electron reduced density matrix within the doubly occupied configuration interaction framework: Treatments of triplet N-electron systems.

In this work, we perform variational calculations of two-electron reduced density matrices corresponding to open-shell N-electron systems within the framework of the doubly occupied configuration interaction treatment, traditionally limited to studies of closed-shell systems. This has allowed us to provide a satisfactory description of molecular systems in triplet states following two methods. One of them adds hydrogen atoms at an infinite distance of the triplet system studied, constituting a singlet supersystem.

Variational determination of ground and excited-state two-electron reduced density matrices in the doubly occupied configuration space: A dispersion operator approach.

This work implements a variational determination of the elements of two-electron reduced density matrices corresponding to the ground and excited states of N-electron interacting systems based on the dispersion operator technique. The procedure extends the previously reported proposal [Nakata et al., J.

Variational determination of the two-electron reduced density matrix within the doubly occupied configuration interaction scheme: An extension to the study of open-shell systems.

This work proposes to describe open-shell molecules or radicals using the framework of the doubly occupied configuration interaction (DOCI) treatments, so far limited to closed-shell system studies. The proposal is based on considering molecular systems in singlet states generated by adding extra hydrogen atoms located at infinite distance from the target radical system. The energy of this radical is obtained by subtracting the energies of the dissociated hydrogen atoms from that provided by the two-electron reduced density matrix corresponding to the singlet state system in the DOCI space, which is variationally calculated by imposing a set of N-representability conditions.

Unrestricted treatment for the direct variational determination of the two-electron reduced density matrix for doubly occupied-configuration-interaction wave functions.

This work extends to the unrestricted orbital approach the procedure described in our previous report [Alcoba et al., J. Chem.

Magnetic Properties of Co(II) Complexes with Polyhedral Carborane Ligands.

In this work we present a computational analysis of a new family of magnetic Co(II) single-ion complexes with large magnetic anisotropy based on icosahedral and octahedral carborane ligands. In particular, we extend our previous computational work on mononuclear Co(II) complexes with 1,2-(HS)-1,2-CBH and 9,12-(HS)-1,2-CBH icosahedral o-carborane ligands to a larger set of complexes where the Co(II) ion is doubly chelated by those ligands and by other two positional isomers belonging to the 1,2-dicarba- closo-dodecaborane family. We also describe Co(II) complexes with octahedral ligands derived from 1,2-dicarba- closo-hexaborane and study the effects of replacing a thiol group by a hydroxy group in both polyhedral geometries, as well as the influence of the position of the carbon atoms.

Variational reduced density matrix method in the doubly occupied configuration interaction space using three-particle -representability conditions.

Ground-state energies and two-particle reduced density matrices (2-RDMs) corresponding to -particle systems are computed variationally within the doubly occupied configuration interaction (DOCI) space by constraining the 2-RDM to satisfy a complete set of three-particle -representability conditions known as three-positivity conditions. These conditions are derived and implemented in the variational calculation of the 2-RDM with standard semidefinite programming algorithms. Ground state energies and 2-RDMs are computed for N, CO, CN, and NO molecules at both equilibrium and nonequilibrium geometries as well as for pairing models at different repulsive interaction strengths.

Magnetic Properties of Mononuclear Co(II) Complexes with Carborane Ligands.

We analyze the magnetic properties of three mononuclear Co(II) coordination complexes using quantum chemical complete active space self-consistent field and N-electron valence perturbation theory approaches. The complexes are characterized by a distorted tetrahedral geometry in which the central ion is doubly chelated by the icosahedral ligands derived from 1,2-(HS)-1,2-CBH (complex I), from 1,2-(HS)-1,2-CBH and 9,12-(HS)-1,2-CBH (complex II), and from 9,12-(HS)-1,2-CBH (complex III), which are two positional isomers of dithiolated 1,2-dicarba- closo-dodecaborane (complex I). Complex I was realized experimentally recently (Tu, D.

Direct variational determination of the two-electron reduced density matrix for doubly occupied-configuration-interaction wave functions: The influence of three-index N-representability conditions.

This work proposes the variational determination of two-electron reduced density matrices corresponding to the ground state of N-electron systems within the doubly occupied-configuration-interaction methodology. The P, Q, and G two-index N-representability conditions have been extended to the T1 and T2 (T2') three-index ones and the resulting optimization problem has been addressed using a standard semidefinite program. We report results obtained from the doubly occupied-configuration-interaction method, from the two-index constraint variational procedure and from the two- and three-index constraint variational treatment.

Conventional Polysomnography Is Not Necessary for the Management of Most Patients with Suspected Obstructive Sleep Apnea. Noninferiority, Randomized Controlled Trial.

Rationale: Home respiratory polygraphy may be a simpler alternative to in-laboratory polysomnography for the management of more symptomatic patients with obstructive sleep apnea, but its effectiveness has not been evaluated across a broad clinical spectrum.

Objectives: To compare the long-term effectiveness (6 mo) of home respiratory polygraphy and polysomnography management protocols in patients with intermediate-to-high sleep apnea suspicion (most patients requiring a sleep study).

Methods: A multicentric, noninferiority, randomized controlled trial with two open parallel arms and a cost-effectiveness analysis was performed in 12 tertiary hospitals in Spain.

Analysis of molecular and (di)atomic dual-descriptor functions and matrices.

In this work, the dual-descriptor is studied in matrix form [Formula: see text] and both coordinates condensed to atoms, resulting in atomic and diatomic (or where applicable, bond) condensed single values. This double partitioning method of the dual-descriptor matrix is proposed within the Hirshfeld-I atoms-in-molecule framework although it is easily extended to other atoms-in-molecules methods. Diagonalizing the resulting atomic and bond dual-descriptor matrices gives eigenvalues and eigenvectors describing the reactivity of atoms and bonds.

Effects of Ethnicity on the Prevalence of Obstructive Sleep Apnoea in Patients with Acute Coronary Syndrome: A Pooled Analysis of the ISAACC Trial and Sleep and Stent Study.

Background: Obstructive sleep apnoea (OSA) is an emerging risk factor for acute coronary syndrome (ACS). We sought to determine the effects of ethnicity on the prevalence of OSA in patients presenting with ACS who participated in an overnight sleep study.

Methods: A pooled analysis using patient-level data from the ISAACC Trial and Sleep and Stent Study was performed.

Spin contamination-free N-electron wave functions in the excitation-based configuration interaction treatment.

This work deals with the spin contamination in N-electron wave functions provided by the excitation-based configuration interaction methods. We propose a procedure to ensure a suitable selection of excited N-electron Slater determinants with respect to a given reference determinant, required in these schemes. The procedure guarantees the construction of N-electron wave functions which are eigenfunctions of the spin-squared operator Sˆ(2), avoiding any spin contamination.

Atom and Bond Fukui Functions and Matrices: A Hirshfeld-I Atoms-in-Molecule Approach.

The Fukui function is often used in its atom-condensed form by isolating it from the molecular Fukui function using a chosen weight function for the atom in the molecule. Recently, Fukui functions and matrices for both atoms and bonds separately were introduced for semiempirical and ab initio levels of theory using Hückel and Mulliken atoms-in-molecule models. In this work, a double partitioning method of the Fukui matrix is proposed within the Hirshfeld-I atoms-in-molecule framework.

Variational Optimization of the Second-Order Density Matrix Corresponding to a Seniority-Zero Configuration Interaction Wave Function.

We perform a direct variational determination of the second-order (two-particle) density matrix corresponding to a many-electron system, under a restricted set of the two-index N-representability P-, Q-, and G-conditions. In addition, we impose a set of necessary constraints that the two-particle density matrix must be derivable from a doubly occupied many-electron wave function, i.e.

Polynomial scaling approximations and dynamic correlation corrections to doubly occupied configuration interaction wave functions.

A class of polynomial scaling methods that approximate Doubly Occupied Configuration Interaction (DOCI) wave functions and improve the description of dynamic correlation is introduced. The accuracy of the resulting wave functions is analysed by comparing energies and studying the overlap between the newly developed methods and full configuration interaction wave functions, showing that a low energy does not necessarily entail a good approximation of the exact wave function. Due to the dependence of DOCI wave functions on the single-particle basis chosen, several orbital optimisation algorithms are introduced.

A hybrid configuration interaction treatment based on seniority number and excitation schemes.

We present a configuration interaction method in which the Hamiltonian of an N-electron system is projected on Slater determinants selected according to the seniority-number criterion along with the traditional excitation-based procedure. This proposed method is especially useful to describe systems which exhibit dynamic (weak) correlation at determined geometric arrangements (where the excitation-based procedure is more suitable) but show static (strong) correlation at other arrangements (where the seniority-number technique is preferred). The hybrid method amends the shortcomings of both individual determinant selection procedures, yielding correct shapes of potential energy curves with results closer to those provided by the full configuration interaction method.