Andrei L. Tchougréeff

Analytic projection from plane-wave and PAW wavefunctions and application to chemical-bonding analysis in solids

Quantum‐chemical computations of solids benefit enormously from numerically efficient plane‐wave (PW) basis sets, and together with the projector augmented‐wave (PAW) method, the latter have risen to one of the predominant standards in computational solid‐state sciences. Despite their advantages, plane waves lack local information, which makes the interpretation of local densities‐of‐states (DOS) difficult and precludes the direct use of atom‐resolved chemical bonding indicators such as the crystal orbital overlap population (COOP) and the crystal orbital Hamilton population (COHP) techniques. Recently, a number of methods have been proposed to overcome this fundamental issue, built around the concept of basis‐set projection onto a local auxiliary basis. In this work, we propose a novel computational technique toward this goal by transferring the PW/PAW wavefunctions to a properly chosen local basis using analytically derived expressions. In particular, we describe a general approach to project both PW and PAW eigenstates onto given custom orbitals, which we then exemplify at the hand of contracted multiple‐ζ Slater‐type orbitals. The validity of the method presented here is illustrated by applications to chemical textbook examples—diamond, gallium arsenide, the transition‐metal titanium—as well as nanoscale allotropes of carbon: a nanotube and the C60 fullerene. Remarkably, the analytical approach not only recovers the total and projected electronic DOS with a high degree of confidence, but it also yields a realistic chemical‐bonding picture in the framework of the projected COHP method.

LOBSTER: A tool to extract chemical bonding from plane-wave based DFT

The computer program LOBSTER (Local Orbital Basis Suite Towards Electronic‐Structure Reconstruction) enables chemical‐bonding analysis based on periodic plane‐wave (PAW) density‐functional theory (DFT) output and is applicable to a wide range of first‐principles simulations in solid‐state and materials chemistry. LOBSTER incorporates analytic projection routines described previously in this very journal [J. Comput. Chem. 2013, 34, 2557] and offers improved functionality. It calculates, among others, atom‐projected densities of states (pDOS), projected crystal orbital Hamilton population (pCOHP) curves, and the recently introduced bond‐weighted distribution function (BWDF). The software is offered free‐of‐charge for non‐commercial research.

Electronic structure and optical spectra of transition metal complexes by the effective Hamiltonian method

SummaryA semiempirical effective Hamiltonian treatment is proposed for transition metal complexes, taking into accountd-electron correlations, weak covalency of the metal-ligand bonds and the electronic structure of the ligand sphere. The technique uses the variation wave function which differs from the usual Hartree-Fock antisymmetrized product of molecular orbitals extended over the whole complex. The scheme is implemented and parameters describing the metal-ligand interactions are adjusted to reproduced-d-excitation spectra of a number of octahedral MF64− (M=Mn, Fe, Co, Ni) anions, Mn(FH)62+ cation, CoCl64− anion, and a tetrahedral CoCl42− anion. The values of the parameters are reasonable, thus confirming the validity of the proposed scheme.

Hydrogen-Bond Networks in Water Clusters (H2O)20: An Exhaustive Quantum-Chemical Analysis

Water aggregates allow for numerous configurations due to different distributions of hydrogen bonds. The total number of possible hydrogen-bond networks is very large even for medium-sized systems. We demonstrate that targeted ultra-fast methods of quantum chemistry make an exhaustive analysis of all configurations possible. The cage of (H(2)O)(20) in the form of the pentagonal dodecahedron is a common motif in water structures. We calculated the spatial and electronic structure of all hydrogen-bond configurations for three systems: idealized cage (H(2)O)(20) and defect cages with one or two hydrogen bonds broken. More than 3 million configurations studied provide unique data on the structure and properties of water clusters. We performed a thorough analysis of the results with the emphasis on the cooperativity in water systems and the structure-property relations.

Ground-State Multiplicities and d- d Excitations of Transition-Metal Complexes by Effective Hamiltonian Method

Magnetic and optical properties of transition-metal complexes are governed by the ground state and the low-energy excitation spectrum of the d-shell of the central transition metal ion. These spectra are successfully fit to the crystal field theory. We present here an account of the effective Hamiltonian method recently developed to calculate the ground state and the excitations of the d-shells of transition-metal complexes and report the results of its application to some complexes of particular interest. 0 1996

HIGH-SPIN-LOW-SPIN TRANSITIONS IN FE(II) COMPLEXES BY EFFECTIVE HAMILTONIAN METHOD

The high-spin-low-spin (HS-LS) transition in iron(II) complexes was studied by the recently developed quantum chemical effective Hamiltonian method. This method uses a trial wave function which is an antisymmetrized product of the fully correlated function of d-electrons and of the Slater determinant of the ligand MOs instead of the conventional Hartree-Fock single determinant trial wave function built of the molecular orbitals spread over an entire complex. This approach allowed us to explicitly take into account the d-electron correlations, the weak covalence of the metal-ligand bonds, and the electronic structure of the ligands. The cooperativity effects in the HS-LS transition occurring in the crystals are briefly discussed and the contribution from the Coulomb forces to the intermolecular interaction responsible for the cooperativity is estimated.

Unconventional Magnetism in a Nitrogen-Containing Analog of Cupric Oxide

We have investigated the magnetic properties of CuNCN, the first nitrogen-based analog of cupric oxide CuO. Our muon-spin relaxation, nuclear magnetic resonance, and electron-spin resonance studies reveal that classical magnetic ordering is absent down to the lowest temperatures. However, a large enhancement of spin correlations and an unexpected inhomogeneous magnetism have been observed below 80 K. We attribute this to a peculiar fragility of the electronic state against weak perturbations due to geometrical frustration, which selects between competing spin-liquid and more conventional frozen states.

Heisenberg Hamiltonian for charge‐transfer organometallic ferromagnets

A highly anisotropic Heisenberg spin one‐half Hamiltonian is derived for the organometallic charge‐transfer ferromagnet DMeFc‐TCNE and its effective exchange parameters are estimated. Its relationship to the generally adopted McConnell picture of ferromagnetic interaction in such systems is established and particular charge‐transfer states responsible for the ferromagnetic sign of the effective spin–spin interaction are discussed. The model proposed is valid for a number of charge‐transfer magnets. Possible effects of high anisotropy on critical temperature in the DMeFc‐TCNE ferromagnet are discussed briefly.

Group functions, Lowdin partition, and hybrid QC/MM methods for large molecular systems

The problem of developing an exact form of the junction between the quantum and classical parts in a hybrid QC/MM approach is considered. We start from the full Hamiltonian for the whole system and assume a specific form of the electron wavefunction, which allows us to separate the electron variables relevant to the reactive (quantum) part of the system from those related to the inert (classical) part. Applying the Lowdin partition to the full Hamiltonian for the molecular system results in general formulae for the potential energy surfaces of a molecular system composed of different parts provided some of these parts are treated quantum mechanically whereas others are treated with use of molecular mechanics. These principles of separating electron variables have been applied to construct an efficient method for analysis of electronic structure and d-electron excitation spectra of transition metal complexes. This method has been also combined with the MM approximation in order to get a description for potential energy surfaces of the complexes and to develop a consistent approach to the known problem of extending molecular mechanics to transition metals.

Hybrid Methods of Molecular Modeling

In this chapter we start with a brief recap of the general setting of the molecular modeling problem and the quantum mechanical and quantum chemical techniques. It may be of interest for students to follow the description of nonstandard tools of quantum mechanics and quantum chemistry presented after that. These tools are then used to develop a general scheme for separating electronic variables in complex molecular systems, which yields the explicit form of its potential energy surface in terms of the electronic structure variables of the subsystem treated at a quantum mechanical level, of the force fields for the subsystem treated classically, and explicitly expressing the central object of any hybrid scheme – the inter-subsystem junction – in terms of the generalized observables of the classically treated subsystem: its one-electron Green’s function and polarization propagator. 1.1. MOTIVATION AND GENERAL SETTING Molecular modeling includes a collection of computer-based tools of varying theoretical soundness, which make it possible to explain, and eventually predict, the properties of molecular systems on the basis of their composition, geometry, and electronic structure. The need for such modeling arises while studying and/or developing various chemical products and/or processes. The raison d’etre of molecular modeling is provided by chemical thermodynamics and chemical kinetics, the basic facts of which are assumed to be known to the reader. According to chemical thermodynamics the relative stability of chemical species and thus their basic capacity to transform to each other (understood in a very wide sense, for example, as the possibility to form solutions i.e. homogeneous mixtures with each other or undergo phase transitions e.g. from gas to liquid state) is described by the equilibrium constant of the (at this point) hypothetical process: R1 +R2 + ... P1 + P2 + ... (1.1) where R1, R2, ... stand for the reactant species and P1, P2, ... stand for the product species governed by the equilibrium constant Keq = Keq(T, P, ...) dependent on The sources in physical chemistry are numerous. Elementary volumes to be known by heart are [1,2]; the more the better.