Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y34: Precision Many Body Physics VIFocus

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Sponsoring Units: DCOMP DAMOP DCMP Chair: Sergei Iskakov, Univ of Michigan  Ann Arbor Room: LACC 409A 
Friday, March 9, 2018 11:15AM  11:51AM 
Y34.00001: Selfenergy embedding theory (SEET) Invited Speaker: Dominika Zgid , Emanuel Gull , Sergei Iskakov , Alexander Rusakov , Lan Tran I will present a discussion of selfenergy embedding theory (SEET) which is a quantum embedding scheme allowing us to describe a chosen subsystem very accurately while keeping the description of the environment at a lower cost. We have applied SEET to both molecular and periodic examples where commonly our chosen subsystem is made out of a set of strongly correlated orbitals while the weakly correlated orbitals constitute an environment. Such a selfenergy separation is very general and to make this procedure applicable to multiple systems a detailed and practical procedure for the evaluation of the system and environment selfenergy is necessary. We list all the intricacies for one of the possible procedures while focusing our discussion on many practical implementation aspects such as the choice of best orbital basis, impurity solver, and many steps necessary to reach high accuracy. 
Friday, March 9, 2018 11:51AM  12:03PM 
Y34.00002: Effect of nodal surface on the geometry of antiferromagnetic iron oxide Joshua Townsend , Luke Shulenburger , Thomas Mattsson , Ken Esler , Ronald Cohen We report quantum variational and diffusion Monte Carlo calculations of antiferromagnetic iron oxide (FeO) which offer a test of the accuracy and predictive power of a single slaterjastrow determinant representation of a highly correlated manybody wave function. 
Friday, March 9, 2018 12:03PM  12:15PM 
Y34.00003: Quantum Monte Carlo Beyond FixedNode/Phase Approximations Using Extended Configuration Spaces and Residuals Lubos Mitas , Cody Melton , Michael Bennett Electronic structure quantum Monte Carlo (QMC) that is based on sampling particle configurations has a number of desired properties such as direct access to interparticle correlations and pointwise information about deviations of the local energy from an estimated eigenvalue. The price for these advantages is the fixednode/phase approximation that is typically used to eliminate the wellknown fermion sign/complex value problems. We explore possibilities of recently introduced spinorbased QMC formulation to find improvements over the fixednode/phase results. One direction is a spinorbased releasednode method that enables one to correct part of the fixednode error by tuning the fixedphase sampling of configurations in overcomplete continuous spin representation. Another strategy is based on the use of residuals froma variational treatment to improve the trial function as well as to increase efficiency of the estimators. An analysis of results from these calculations enables us to identify the types of fixednode biases commonly present in trial functions and to estimate corresponding improvements in accuracy and efficiency. 
Friday, March 9, 2018 12:15PM  12:27PM 
Y34.00004: Multideterminant diffusion Monte Carlo applied to Molecules and Solids Anouar Benali , Thomas Applencourt , Jeongnim Kim , Anthony Scemama , Michel Caffarel In the past decade, fixednode Diffusion Monte Carlo using a single SlaterJastrow determinant as a trial wavefunction has proven to reproduce systematically (within 50meV) the energies of a wide range of molecules and solids. While it has been demonstrated for molecular systems that the path to chemical accuracy (<1Kcal/mol) relys on improving the accuracy of the nodal surface, which can be achieved using a multideterminants trial wavefunction from MCSCF or Selected CI, no such calculations have been attempted on a solid. In this talk, we will discuss the effect of using a multideterminant selected CI trial wavefunctions, generated with Configuration Interaction using a Perturbative Selection made Iteratively (CIPSI),in diffusion Monte Carlo (DMC) on the energies of molecules and solids. This first study will allow us to analyze the feasibility, advantages and size consistency issues when using large multideterminants expansion in QMC at the era of exascale computers. 
Friday, March 9, 2018 12:27PM  12:39PM 
Y34.00005: Improving the Accuracy of AFQMC with NonOrthogonal MultiDeterminant Wave Functions Edgar Landinez Borda , Matthew Otten , Cyrus Umrigar , Miguel Morales We explore the use of trial wave functions in AuxiliaryField Quantum Monte Carlo constructed from nonorthogonal Slater determinant expansions. The use of these wave functions leads to systematically increased accuracy with a reduced number of terms and a compact representation, when compared to traditional determinant expansions constructed from particlehole excitations. We demonstrate significantly improved accuracy when compared to traditional trial wavefunctions, as well as systematically improvable results in both strongly correlated molecular calculations and periodic systems. First, we compute the isomerization path of the [Cu_{2}O_{2}]^{+2} molecule and compare the profile of relative energies along the path against DMRG, CRCC and SPHF calculations. This problem is well known because the accurate calculation of the correlation energy along the path is challenging. The present methodology exhibits versatility and good agreement using only several tens to hundreds of determinants. We also test the methodology in a periodic system performing calculations for Silicon in the diamond phase. We compare against Coupled Cluster and converged selected CI calculations. As shown, the method is capable of reaching submHa accuracy even when a modest number of determinant is used. 
Friday, March 9, 2018 12:39PM  12:51PM 
Y34.00006: Ab Initio Finite Temperature Auxiliary Field Quantum Monte Carlo Yuan Liu , Brenda Rubenstein Predicting finite temperature (FT) properties of molecules and solids is critical to understanding many physical and chemical processes. However, developing accurate, yet efficient theoretical approaches for FT applications remains an outstanding challenge. Aside from FT mean field theories that have limited accuracy and FT Full Configuration Interaction with exponential scaling, proper ways to generalize various post HartreeFock theories to FT is still an open problem. Furthermore, FT generalizations of Density Functional Theory, considered to be promising candidates for future large scale simulation, require additional benchmarking against more accurate methods. 
Friday, March 9, 2018 12:51PM  1:03PM 
Y34.00007: Accelerating the use of multideterminant trial wave functions in auxiliaryfield quantum Monte Carlo calculations Hao Shi , Shiwei Zhang The trial wave function is used in auxiliaryfield quantum Monte Carlo (AFQMC) method to initialize the projection, control the sign/phase problem, and serve as the starting point for backpropagation. Typically a single determinant wave function is used. The computation and memory costs of using multideterminant trial wave functions tends to grow linearly with the number of Slater determinants. An efficient algorithm is proposed for AFQMC simulations with configuration interaction trial wave functions. Large numbers of Slater determinants can be reached with much smaller memory usage and computation time. Systematic tests can be made by increasing the number of determinants in the trial wave function. Thousands to millions of determinants can be reached in molecular systems. The error from the approximation to control the sign/phase problem often becomes negligible with the new capability, and the Monte Carlo fluctuation can be much reduced. We illustrate this with examples on atoms and molecules, including challenging transition metal systems. 
Friday, March 9, 2018 1:03PM  1:15PM 
Y34.00008: NonOrthogonal Determinant MultiSlaterJastrow Wave Functions in QMC Shivesh Pathak , Lucas Wagner The efficiency of abinitio quantum Monte Carlo (QMC) algorithms benefits greatly from compact variational trial wave functions that accurately reproduce ground state properties of a system. We investigate the possibility of using nonorthogonal determinants to create more compact wave functions than standard multiSlaterJastrow trial wave functions. As a test case, we compute variational and diffusion Monte Carlo (DMC) energies of a C_{2} molecule. For a given multideterminant expansion, we find that allowing the determinants to be nonorthogonal results in a fairly consistent ~ 0.4 eV improvement in the variational energy and ~ 0.2 eV improvement in the DMC energy. Our calculations indicate that trial wave functions with nonorthogonal determinants may noticeably improve computed energies in a QMC calculation when compared to their traditional orthogonal counterparts. 
Friday, March 9, 2018 1:15PM  1:27PM 
Y34.00009: Automated Construction of U(1)invariant MatrixProduct Operators from Graph Representations Sebastian Paeckel , Thomas Koehler , Salvatore Manmana We present an algorithmic construction scheme for matrixproductoperator (MPO) representations of arbitrary U(1)invariant operators in case a finitestatesmachine (FSM) formulation exists. The method automatizes two major construction steps: 
Friday, March 9, 2018 1:27PM  1:39PM 
Y34.00010: Magnetism and metalinsulator transition in nickel oxide from AuxiliaryField Quantum Monte Carlo (AFQMC) studies Shuai Zhang , Fionn Malone , Miguel Morales NiO is of great interests in condensed matter physics. Within single particle meanfield framework, one can obtain its insulating antiferromagnetic ground state and correct band gap, magnetic momentum, or lattice constant by adjusting exchange correlation interactions. However, it is challenging to reproduce all the properties using the same setup, and the predicted magnetic and metallic transitions are questionable. It demands physically accurate and computationally efficient manybody algorithms to tackle these questions. AFQMC works in second quantized representation and has been shown promising in solving extended systems with high accuracy [comparable to those of CCSD(T) and FCI] and low scaling (N^{3}N^{4}). As an integral part of the Center for Predictive Simulation of Functional Materials (CPSFM), we develop efficient algorithms based on AFQMC and massively parallel supercomputer platforms and study transition metal oxides. We control the phase and sign problem with the phaseless formalism. As an example, we calculate the energy and magnetism of NiO in different magnetic states and upon volume changes. Our results agree well with experiments and can benchmark future theoretical investigations. 
Friday, March 9, 2018 1:39PM  1:51PM 
Y34.00011: Eightvalence electron configurationinteraction manybody perturbation theory calculations of noblegas atoms Igor Savukov Excited states of noblegas atoms present certain difficulties to configurationinteraction manybody perturbation theory (CIMBPT). One specific problem is that in formalism of particlehole CIMBPT, the perturbation theory for hole states does not converge well. One solution was proposed to use either allorder methods [1] or to modify MBPT denominators [2]. In the current work, on example of Ar, we find that this problem can be solved by treating 3s and 3p electrons as valence electrons (8 total), and by including the residual interactions with 1s, 2s, 2p core electrons via MBPT. The configuration space grows very quickly with the number of allowable excitations, so we impose certain constrains on them, for example by excluding triplet excitations, to have manageable CI space. We find that with our approach we can obtain energies, g factors, and matrix elements in reasonable agreement with experiments and other calculations. 
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