Faculty & Staff Researchhttp://hdl.handle.net/1911/751722017-09-22T05:20:36Z2017-09-22T05:20:36ZSpin-projected generalized Hartree-Fock method as a polynomial of particle-hole excitationsHenderson, Thomas M.Scuseria, Gustavo E.http://hdl.handle.net/1911/974102017-09-20T08:01:51Z2017-01-01T00:00:00ZSpin-projected generalized Hartree-Fock method as a polynomial of particle-hole excitations
Henderson, Thomas M.; Scuseria, Gustavo E.
The past several years have seen renewed interest in the use of symmetry-projected Hartree-Fock for the description of strong correlations. Unfortunately, these symmetry-projected mean-field methods do not adequately account for dynamic correlation. Presumably, this shortcoming could be addressed if one could combine symmetry-projected Hartree-Fock with a many-body method such as coupled-cluster theory, but this is by no means straightforward because the two techniques are formulated in very different ways. However, we have recently shown that the singlet
S
2
-projected unrestricted Hartree-Fock wave function can in fact be written in a coupled-cluster-like wave function. That is, the spin-projected unrestricted Hartree-Fock wave function can be written as a polynomial of a double-excitation operator acting on some closed-shell reference determinant. Here, we extend this result and show that the spin-projected generalized Hartree-Fock wave function (which has both
S
2
and
S
z
projection) is likewise a polynomial of low-order excitation operators acting on a closed-shell determinant and provide a closed-form expression for the resulting polynomial coefficients. The spin projection of the generalized Hartree-Fock wave function introduces connected triple and quadruple excitations which are absent when spin-projecting an unrestricted Hartree-Fock determinant. We include a few preliminary applications of the combination of this spin-projected Hartree-Fock and coupled-cluster theory to the Hubbard Hamiltonian and comment on generalizations of the methodology. Results here are not for production level, but a similarity-transformed theory that combines the two offers the promise of being accurate for both weak and strong correlation, and may offer significant improvements in the intermediate correlation regime where neither projected Hartree-Fock nor coupled cluster is particularly accurate.
2017-01-01T00:00:00ZWhole-Genome Sequence of the 1,4-Dioxane-Degrading Bacterium Mycobacterium dioxanotrophicus PH-06He, YaWei, KangfeiSi, KaiweiMathieu, JacquesLi, MengyanAlvarez, Pedro J.J.http://hdl.handle.net/1911/974112017-09-20T08:01:52Z2017-01-01T00:00:00ZWhole-Genome Sequence of the 1,4-Dioxane-Degrading Bacterium Mycobacterium dioxanotrophicus PH-06
He, Ya; Wei, Kangfei; Si, Kaiwei; Mathieu, Jacques; Li, Mengyan; Alvarez, Pedro J.J.
We report here the complete genome sequence of Mycobacterium dioxanotrophicus PH-06, which is capable of using 1,4-dioxane as a sole source of carbon and energy. The reported sequence will enable the elucidation of this novel metabolic pathway and the development of molecular biomarkers to assess bioremediation potential at contaminated sites.
2017-01-01T00:00:00ZFull-waveform inversion via source-receiver extensionHuang, GuanghuiNammour, RamiSymes, Williamhttp://hdl.handle.net/1911/974062017-09-20T08:01:50Z2017-01-01T00:00:00ZFull-waveform inversion via source-receiver extension
Huang, Guanghui; Nammour, Rami; Symes, William
Full-waveform inversion produces highly resolved images of the subsurface and quantitative estimation of seismic wave velocity, provided that its initial model is kinematically accurate at the longest data wavelengths. If this initialization constraint is not satisfied, iterative model updating tends to stagnate at kinematically incorrect velocity models producing suboptimal images. The source-receiver extension overcomes this “cycle-skip” pathology by modeling each trace with its own proper source wavelet, permitting a good data fit throughout the inversion process. Because source wavelets should be constant (or vary systematically) across a shot gather, a measure of source trace dependence, for example, the mean square of the signature-deconvolved wavelet scaled by time lag, can be minimized to update the velocity model. For kinematically simple data, such measures of wavelet variance are mathematically equivalent to traveltime misfit. Thus, the model obtained by source-receiver extended inversion is close to that produced by traveltime tomography, even though the process uses no picked times. For more complex data, in which energy travels from source to receiver by multiple raypaths, Green’s function spectral notches may lead to slowly decaying trace-dependent wavelets with energy at time lags unrelated to traveltime error. Tikhonov regularization of the data-fitting problem suppresses these large-lag signals. Numerical examples suggest that this regularized formulation of source-receiver extended inversion is capable of recovering reasonably good velocity models from synthetic transmission and reflection data without stagnation at suboptimal models encountered by standard full-waveform inversion, but with essentially the same computational cost.
2017-01-01T00:00:00ZDynamic zero modes of Dirac fermions and competing singlet phases of antiferromagnetic orderGoswami, PallabSi, Qimiaohttp://hdl.handle.net/1911/974042017-09-20T08:01:45Z2017-01-01T00:00:00ZDynamic zero modes of Dirac fermions and competing singlet phases of antiferromagnetic order
Goswami, Pallab; Si, Qimiao
In quantum spin systems, singlet phases often develop in the vicinity of an antiferromagnetic order. Typical settings for such problems arise when itinerant fermions are also present. In this paper, we develop a theoretical framework for addressing such competing orders in an itinerant system, described by Dirac fermions strongly coupled to an O(3) nonlinear sigma model. We focus on two spatial dimensions, where upon disordering the antiferromagnetic order by quantum fluctuations the singular tunneling events also known as (anti)hedgehogs can nucleate competing singlet orders in the paramagnetic phase. In the presence of an isolated hedgehog configuration of the nonlinear sigma model field, we show that the fermion determinant vanishes as the dynamic Euclidean Dirac operator supports fermion zero modes of definite chirality. This provides a topological mechanism for suppressing the tunneling events. Using the methodology of quantum chromodynamics, we evaluate the fermion determinant in the close proximity of magnetic quantum phase transition, when the antiferromagnetic order-parameter field can be described by a dilute gas of hedgehogs and antihedgehogs. We show how the precise nature of emergent singlet order is determined by the overlap between dynamic fermion zero modes of opposite chirality, localized on the hedgehogs and antihedgehogs. For a Kondo-Heisenberg model on the honeycomb lattice, we demonstrate the competition between spin Peierls order and Kondo singlet formation, thereby elucidating its global phase diagram. We also discuss other physical problems that can be addressed within this general framework.
2017-01-01T00:00:00Z