Electron Pair Methods vs. Independent Particle Approximation: Quasiparticle Transformations

Some basic algebraic features of quasiparticle transformations are reviewed. Special nonlinear quasiparticle transformations are introduced leading to the second quantized counterparts of gerninal-type (correlated) wave functions. Algebraic representations of strong and weak orthogonality are discussed, and these issues are generalized to the case of non-orthogonal basis sets leading to the concepts of strong and weak biorthogonaltty

The Representation of the Chemical Bond in Quantum Chemical Calculations

The mathematical representation of chemical bonds in molecules is discussed. The molecule is viewed as a system of weakly interacting chemical bonds. The intrabond problems and the averaged electrostatic interbond interactions are handled at the . zeroth order, while the small interbond delocalization and dispersion effects are taken into account on the basis of the perturbation theory. A special diagrammatic technique is applied for obtaining delocalization corrections for strictly localized orbitals. A general second quantized theory is discussed in which the chemical bonds are identified with two-electron local bond structures characterized by composite-particle creation operators showing Bose-type commutation rules. This approach accounts for intrabond correlation already at the zeroth order

Conformation Analysis in Light of Localization and Delocalization

The role of electron delocalization in conformational effects, especially in giving rise to barrier forces is discussed in the bond orbital framework. Using orthogonal bond orbitals, the effects of through space and through bond delocalization interactions is demonstrated;numerical examples show the predominant role of through space delocalization. The total energy obtained by strictly localized orthogonal bond orbitals is shown to be rather independent of the relative orientations of the bonds. Second order perturbative delocalization energy corrections are interpreted as bond- bond pair potentials within the orthogonal basis. On the contrary, nonorthogonal bond orbitals lead to an energy expression which is very sensitive to the bond orientations even if one neglects completelyelectron delocalization. The origin of the barriers is discussed in terms of nonempirical bond-bond pair potentials

Conformation Analysis in Light of Localization and Delocalization

The role of electron delocalization in conformational effects, especially in giving rise to barrier forces is discussed in the bond orbital framework. Using orthogonal bond orbitals, the effects of through space and through bond delocalization interactions is demonstrated;numerical examples show the predominant role of through space delocalization. The total energy obtained by strictly localized orthogonal bond orbitals is shown to be rather independent of the relative orientations of the bonds. Second order perturbative delocalization energy corrections are interpreted as bond- bond pair potentials within the orthogonal basis. On the contrary, nonorthogonal bond orbitals lead to an energy expression which is very sensitive to the bond orientations even if one neglects completelyelectron delocalization. The origin of the barriers is discussed in terms of nonempirical bond-bond pair potentials

The Representation of the Chemical Bond in Quantum Chemical Calculations

The mathematical representation of chemical bonds in molecules is discussed. The molecule is viewed as a system of weakly interacting chemical bonds. The intrabond problems and the averaged electrostatic interbond interactions are handled at the . zeroth order, while the small interbond delocalization and dispersion effects are taken into account on the basis of the perturbation theory. A special diagrammatic technique is applied for obtaining delocalization corrections for strictly localized orbitals. A general second quantized theory is discussed in which the chemical bonds are identified with two-electron local bond structures characterized by composite-particle creation operators showing Bose-type commutation rules. This approach accounts for intrabond correlation already at the zeroth order

Double Time Window Targeting Technique: Real time DMRG dynamics in the PPP model

We present a generalized adaptive time-dependent density matrix renormalization group (DMRG) scheme, called the {\it double time window targeting} (DTWT) technique, which gives accurate results with nominal computational resources, within reasonable computational time. This procedure originates from the amalgamation of the features of pace keeping DMRG algorithm, first proposed by Luo {\it et. al}, [Phys.Rev. Lett. {\bf 91}, 049701 (2003)], and the time-step targeting (TST) algorithm by Feiguin and White [Phys. Rev. B {\bf 72}, 020404 (2005)]. Using the DTWT technique, we study the phenomena of spin-charge separation in conjugated polymers (materials for molecular electronics and spintronics), which have long-range electron-electron interactions and belong to the class of strongly correlated low-dimensional many-body systems. The issue of real time dynamics within the Pariser-Parr-Pople (PPP) model which includes long-range electron correlations has not been addressed in the literature so far. The present study on PPP chains has revealed that, (i) long-range electron correlations enable both the charge and spin degree of freedom of the electron, to propagate faster in the PPP model compared to Hubbard model, (ii) for standard parameters of the PPP model as applied to conjugated polymers, the charge velocity is almost twice that of the spin velocity and, (iii) the simplistic interpretation of long-range correlations by merely renormalizing the {\it U} value of the Hubbard model fails to explain the dynamics of doped holes/electrons in the PPP model.Comment: Final (published) version; 39 pages, 13 figures, 1 table; 2 new references adde

Comparison of low-order multireference many-body perturbation theories

Tests have been made to benchmark and assess the relative accuracies of low-order multireference perturbation theories as compared to coupled cluster (CC) and full configuration interaction (FCI) methods. Test calculations include the ground and some excited states of the Be, H2, BeH2, CH2, and SiH2 systems. Comparisons with FCI and CC calculations show that in most cases the effective valence shell Hamiltonian (Hv) method is more accurate than other low-order multireference perturbation theories, although none of the perturbative methods is as accurate as the CC approximations. We also briefly discuss some of the basic differences among the multireference perturbation theories considered in this work