Localized Hartree Fock Method Research Articles

The role of exact‐exchange in the theoretical description of organic‐metal interfaces

AbstractWe investigate the ability of different density functional methods to describe the electronic properties of isolated gold clusters, self‐assembled monolayers (SAM) of oligophenylthiols (including the depolarization effect), and the biphenylthiol/gold interface. To elucidate the role of the exchange interaction, we consider a hierarchy of functionals including conventional (e.g., within the gradient corrected approximation), hybrid, and effective exact‐exchange functionals, namely the Localized Hartree–Fock (LHF) method, which is free from the self‐interaction‐error (SIE).We find that conventional exchange‐correlation functionals cannot well describe the energy‐level alignment at the metal/organic interface and predict a negligible metal‐molecule charge‐transfer. In addition, an overestimation of dipole moments and polarization effects are obtained in oligophenylthiols, leading to a wrong description of the SAM depolarization effect. Both limitations are mostly overcome if exact‐exchange contributions are taken into account either using an hybrid functional or the LHF method. In particular, an accurate description of the metal/organic interface is only achieved using SIE free methods. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Structural and electronic properties of gold microclusters: assessment of the localized Hartree–Fock method

We assess the localized Hartree-Fock (LHF) method for the simulation of the structural and electronic properties of gold microclusters. We compute the minimum-energy geometries, atomization energies, dipole moments, and single-particle spectra for five gold clusters, Au(N), and their anions, Au(N)-, with N < or = 4. Calculations are performed with the LHF functional, with and without the addition of the Lee-Yang-Parr (LYP) correlation, and the results are compared with those obtained from other semilocal and hybrid functionals as well as from Hartree-Fock calculations. The LHF functional yields structural properties of similar quality to the other exchange-only methods, but superior molecular orbital energies are found. The LHF single-particle spectrum is free of the artifacts typical for HF and standard DFT calculations and in good agreement with available experimental reference data. The inclusion of correlation by the LYP functional produces a significant improvement of geometries and energetics, but has only a minor impact on the orbital energies.

The localized Hartree–Fock method for a self-interaction free Kohn–Sham potential: applications to closed and open-shell molecules

The localized Hartree–Fock and the open-shell LHF (OSLHF) approaches are reviewed and rederived under a unique formalism. Three different treatments of the OSLHF correction term are discussed and results for excitation energies are presented for small- and medium-size closed- and open-shell molecules.

Torsional potential of π-conjugated molecules using the localized Hartree–Fock Kohn–Sham exchange potential

We have assessed the reliability of the localized Hartree–Fock (LHF) Kohn–Sham exchange potential for the description of the torsional potential of π-conjugated molecules. The LHF method is compared with other exchange-only energy functionals, as well as experiments and ab initio reference calculations for butadiene, biphenyl and bithiophene. Results indicate that the LHF method provides a good and system-independent description of the torsional potential of π-conjugated molecules and it is the only functional that can be improved by the addition of a correlation term. Using the Lee–Yang–Parr correlation functional very good agreement with reference data is found.

Open-shell localized Hartree–Fock method based on the generalized adiabatic connection Kohn–Sham formalism for a self-consistent treatment of excited states

An effective exact-exchange Kohn-Sham approach for the treatment of excited electronic states, the generalized adiabatic connection open-shell localized Hartree-Fock (GAC-OSLHF) method is presented. The GAC-OSLHF method is based on the generalized adiabatic connection Kohn-Sham formalism and therefore capable of treating excited electronic states, which are not the energetically lowest of their symmetry. The method is self-interaction free and allows for a fully self-consistent computation of excited valence as well as Rydberg states. Results for atoms and small- and medium-size molecules are presented and compared to restricted open-shell Hartree-Fock (ROHF) and time-dependent density-functional results as well as to experimental data. While GAC-OSLHF and ROHF results are quite close to each other, the GAC-OSLHF method shows a much better convergence behavior. Moreover, the GAC-OSLHF method as a Kohn-Sham method, in contrast to the ROHF approach, represents a framework which allows also for a treatment of correlation besides an exchange by appropriate functionals. In contrast to the common time-dependent density-functional methods, the GAC-OSLHF approach is capable of treating doubly or multiply excited states and can be easily applied to molecules with an open-shell ground state. On the nodal planes of the energetically highest occupied orbital, the local multiplicative GAC-OSLHF exchange potential asymptotically approaches a different, i.e., nonzero, value than in other regions, an asymptotic behavior which is known from exact Kohn-Sham exchange potentials of ground states of molecules.

Electronic and optical properties of functionalized carbon chains with the localized Hartree–Fock and conventional Kohn–Sham methods

Abstract The electronic and optical properties of extended functionalized carbyne chains, polyynes and cumulenes, are investigated with the localized Hartree–Fock method, with conventional Kohn–Sham methods, and with the Hartree–Fock method. It is found that even for very long polyynes the carbon–carbon bond lengths within a polyyne alternate while for long cumulenes no carbon–carbon bond length alternation occurs. Polyynes exhibit a finite HOMO–LUMO gap even if they become very long while cumulenes are found to become metallic in the limit of long chain lengths. The geometry and the electro-optical properties of polyynes cannot be influenced significantly by simple sp-σ-bonded end groups. The optically active 1 Σ u + ←X 1 Σ g + electronic transition in polyynes is investigated by time-dependent density-functional theory (TDDFT). The known systematic underestimation of excitation energies in large chain-like systems by TDDFT methods is also found for the systems considered here. Deficiencies in the commonly used exchange-correlation kernels are identified as the main source of this shortcoming of TDDFT methods. Unphysical Coulomb self-interactions present in conventional Kohn–Sham potentials seem to not contribute significantly to the problem.

Density-functional calculations of NMR shielding constants using the localized Hartree–Fock method

Density-functional calculations of NMR shielding constants using the localized Hartree–Fock method and a local version of the B3LYP functional, LB3LYP, are presented and compared with traditional density-functional and wavefunction-based methods, as well as to experiment. The localized Hartree–Fock potential, being free of unphysical self-interactions and empirical parameters, shows a significantly better performance than traditional schemes based on generalized gradient approximations and hybrid functionals. The localized B3LYP functional improves on the parent B3LYP and BLYP functionals, and is suitable for combined thermochemical and spectroscopic investigations.

The Kohn–Sham treatment of anions via the localized Hartree–Fock method

Kohn–Sham calculations of small anions with the localized Hartree–Fock (LHF) method, a recently developed effective exact exchange Kohn–Sham (KS) method, are presented. In contrast to conventional KS methods employing density-functionals from the local density or from generalized gradient approximations, the LHF approach yields bound anions, and thus, is found to be suitable for the treatment of anions. The superiority of the LHF method over conventional KS approaches originates in the fact that the former, in contrast to the latter, is free of Coulomb self-interaction. This is demonstrated by a discussion of the involved exchange and effective KS potentials.

First-principles study of the rocksalt–cesium chloride relative phase stability in alkali halides

We present a detailed investigation of observable properties associated with the relative stability of the rocksalt ( B1) and cesium chloride (B2) phases in the AX (A5Li, Na, K, Rb, Cs; X5F, Cl, Br, I! crystal family. Thermodynamic B1!B2 transition pressures and DY 5Y (B2)2Y (B1) differences in total energies, volumes, and bulk moduli at zero and transition pressures are computed following a localized Hartree-Fock method. The arrangement of the data in clear trends is shown to be mainly dominated by the cation atomic number. This behavior is well interpreted in terms of a variety of microscopic arguments that emerge from ~i! the evaluation of the energy Hessian at the B1 and B2 points and ~ii! the decomposition of the energy and pressure in anionic and cationic classical and quantum-mechanical contributions.

The asymptotic region of the Kohn–Sham exchange potential in molecules

The Kohn–Sham exchange potential is shown to approach on nodal surfaces of the energetically highest occupied orbital different asymptotic limits than in other regions. This leads to strong anisotropies and barrier–well structures in the near-asymptotic region. Effective exact-exchange potentials, like the one obtained in the recently introduced localized Hartree–Fock method, exhibit the correct asymptotic limits and the accompanying structures in the near-asymptotic region. An efficient, accurate method for the calculation of the Slater potential and of effective exact-exchange potentials in the asymptotic region is presented. The method is based on an asymptotic continuation of Kohn–Sham (KS) orbitals and is numerically stable up to arbitrary large distances from the molecule. It can easily be implemented in existing quantum chemistry codes employing Gaussian basis sets. The asymptotic region of effective exact-exchange potentials is shown to be different from the various asymptotically corrected exchange-correlation functionals and to strongly affect unoccupied KS orbitals. Results for small- and medium-size molecules are presented.