The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
C J Umrigar - One of the best experts on this subject based on the ideXlab platform.
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fast semistochastic heat bath Configuration Interaction
Journal of Chemical Physics, 2018Co-Authors: Matthew Otten, Adam A Holmes, Sandeep Sharma, C J UmrigarAbstract:This paper presents in detail our fast semistochastic heat-bath Configuration Interaction (SHCI) method for solving the many-body Schrodinger equation. We identify and eliminate computational bottlenecks in both the variational and perturbative steps of the SHCI algorithm. We also describe the parallelization and the key data structures in our implementation, such as the distributed hash table. The improved SHCI algorithm enables us to include in our variational wavefunction two orders of magnitude more determinants than has been reported previously with other selected Configuration Interaction methods. We use our algorithm to calculate an accurate benchmark energy for the chromium dimer with the X2C relativistic Hamiltonian in the cc-pVDZ-DK basis, correlating 28 electrons in 76 spatial orbitals. Our largest calculation uses two billion Slater determinants in the variational space and semistochastically includes perturbative contributions from at least trillions of additional determinants with better than 10-5 Ha statistical uncertainty.
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fast semistochastic heat bath Configuration Interaction
arXiv: Chemical Physics, 2018Co-Authors: Matthew Otten, Adam A Holmes, Sandeep Sharma, C J UmrigarAbstract:This paper presents in detail our fast semistochastic heat-bath Configuration Interaction (SHCI) method for solving the many-body Schrodinger equation. We identify and eliminate computational bottlenecks in both the variational and perturbative steps of the SHCI algorithm. We also describe the parallelization and the key data structures in our implementation, such as the distributed hash table. The improved SHCI algorithm enables us to include in our variational wavefunction two orders of magnitude more determinants than has been reported previously with other selected Configuration Interaction methods. We use our algorithm to calculate an accurate benchmark energy for the chromium dimer with the X2C relativistic Hamiltonian in the cc-pVDZ-DK basis, correlating 28 electrons in 76 spatial orbitals. Our largest calculation uses two billion Slater determinants in the variational space, and semistochastically includes perturbative contributions from at least trillions of additional determinants with better than 10 microhartree statistical uncertainty.
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excited states of methylene polyenes and ozone from heat bath Configuration Interaction
Journal of Physical Chemistry A, 2018Co-Authors: Alan D Chien, C J Umrigar, Matthew Otten, Adam A Holmes, Sandeep Sharma, Paul M ZimmermanAbstract:The electronically excited states of methylene (CH2), ethylene (C2H4), butadiene (C4H6), hexatriene (C6H8), and ozone (O3) have long proven challenging due to their complex mixtures of static and dynamic correlations. The semistochastic heat-bath Configuration Interaction (SHCI) algorithm, which efficiently and systematically approaches the full Configuration Interaction (FCI) limit, is used to provide close approximations to the FCI energies in these systems. This article presents the largest FCI-level calculation to date on hexatriene, using a polarized double-ζ basis (ANO-L-pVDZ), which gives rise to a Hilbert space containing more than 1038 determinants. These calculations give vertical excitation energies of 5.58 and 5.59 eV, respectively, for the 21Ag and 11Bu states, showing that they are nearly degenerate. The same excitation energies in butadiene/ANO-L-pVDZ were found to be 6.58 and 6.45 eV. In addition to these benchmarks, our calculations strongly support the presence of a previously hypothesiz...
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excited states using semistochastic heat bath Configuration Interaction
Journal of Chemical Physics, 2017Co-Authors: Adam A Holmes, C J Umrigar, Sandeep SharmaAbstract:We extend our recently developed heat-bath Configuration Interaction (HCI) algorithm, and our semistochastic algorithm for performing multireference perturbation theory, to calculate excited-state wavefunctions and energies. We employ time-reversal symmetry, which reduces the memory requirements by more than a factor of two. An extrapolation technique is introduced to reliably extrapolate HCI energies to the full CI limit. The resulting algorithm is used to compute fourteen low-lying potential energy surfaces of the carbon dimer using the cc-pV5Z basis set, with an estimated error in energy of 30-50 μHa compared to full CI. The excitation energies obtained using our algorithm have a mean absolute deviation of 0.02 eV compared to experimental values.
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excited states using semistochastic heat bath Configuration Interaction
arXiv: Strongly Correlated Electrons, 2017Co-Authors: Adam A Holmes, C J Umrigar, Sandeep SharmaAbstract:We extend our recently-developed heat-bath Configuration Interaction (HCI) algorithm, and our semistochastic algorithm for performing multireference perturbation theory, to the calculation of excited-state wavefunctions and energies. We employ time-reversal symmetry, which reduces the memory requirements by more than a factor of two. An extrapolation technique is introduced to reliably extrapolate HCI energies to the Full CI limit. The resulting algorithm is used to compute the twelve lowest-lying potential energy surfaces of the carbon dimer using the cc-pV5Z basis set, with an estimated error in energy of 30-50 {\mu}Ha compared to Full CI. The excitation energies obtained using our algorithm have a mean absolute deviation of 0.02 eV compared to experimental values. We also calculate the complete active-space (CAS) energies of the S0, S1, and T0 states of tetracene, which are of relevance to singlet fission, by fully correlating active spaces as large as 18 electrons in 36 orbitals.
Sandeep Sharma - One of the best experts on this subject based on the ideXlab platform.
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multireference Configuration Interaction and perturbation theory without reduced density matrices
Journal of Chemical Physics, 2019Co-Authors: Ankit Mahajan, Nick S Blunt, Iliya Sabzevari, Sandeep SharmaAbstract:The computationally expensive evaluation and storage of high-rank reduced density matrices (RDMs) has been the bottleneck in the calculation of dynamic correlation for multireference wave functions in large active spaces. We present a stochastic formulation of multireference Configuration Interaction and perturbation theory that avoids the need for these expensive RDMs. The algorithm presented here is flexible enough to incorporate a wide variety of active space reference wave functions, including selected Configuration Interaction, matrix product states, and symmetry-projected Jastrow mean field wave functions. It enjoys the usual attractive features of Monte Carlo methods, such as embarrassing parallelizability and low memory costs. We find that the stochastic algorithm is already competitive with the deterministic algorithm for small active spaces, containing as few as 14 orbitals. We illustrate the utility of our stochastic formulation using benchmark applications.
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fast semistochastic heat bath Configuration Interaction
Journal of Chemical Physics, 2018Co-Authors: Matthew Otten, Adam A Holmes, Sandeep Sharma, C J UmrigarAbstract:This paper presents in detail our fast semistochastic heat-bath Configuration Interaction (SHCI) method for solving the many-body Schrodinger equation. We identify and eliminate computational bottlenecks in both the variational and perturbative steps of the SHCI algorithm. We also describe the parallelization and the key data structures in our implementation, such as the distributed hash table. The improved SHCI algorithm enables us to include in our variational wavefunction two orders of magnitude more determinants than has been reported previously with other selected Configuration Interaction methods. We use our algorithm to calculate an accurate benchmark energy for the chromium dimer with the X2C relativistic Hamiltonian in the cc-pVDZ-DK basis, correlating 28 electrons in 76 spatial orbitals. Our largest calculation uses two billion Slater determinants in the variational space and semistochastically includes perturbative contributions from at least trillions of additional determinants with better than 10-5 Ha statistical uncertainty.
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fast semistochastic heat bath Configuration Interaction
arXiv: Chemical Physics, 2018Co-Authors: Matthew Otten, Adam A Holmes, Sandeep Sharma, C J UmrigarAbstract:This paper presents in detail our fast semistochastic heat-bath Configuration Interaction (SHCI) method for solving the many-body Schrodinger equation. We identify and eliminate computational bottlenecks in both the variational and perturbative steps of the SHCI algorithm. We also describe the parallelization and the key data structures in our implementation, such as the distributed hash table. The improved SHCI algorithm enables us to include in our variational wavefunction two orders of magnitude more determinants than has been reported previously with other selected Configuration Interaction methods. We use our algorithm to calculate an accurate benchmark energy for the chromium dimer with the X2C relativistic Hamiltonian in the cc-pVDZ-DK basis, correlating 28 electrons in 76 spatial orbitals. Our largest calculation uses two billion Slater determinants in the variational space, and semistochastically includes perturbative contributions from at least trillions of additional determinants with better than 10 microhartree statistical uncertainty.
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excited states of methylene polyenes and ozone from heat bath Configuration Interaction
Journal of Physical Chemistry A, 2018Co-Authors: Alan D Chien, C J Umrigar, Matthew Otten, Adam A Holmes, Sandeep Sharma, Paul M ZimmermanAbstract:The electronically excited states of methylene (CH2), ethylene (C2H4), butadiene (C4H6), hexatriene (C6H8), and ozone (O3) have long proven challenging due to their complex mixtures of static and dynamic correlations. The semistochastic heat-bath Configuration Interaction (SHCI) algorithm, which efficiently and systematically approaches the full Configuration Interaction (FCI) limit, is used to provide close approximations to the FCI energies in these systems. This article presents the largest FCI-level calculation to date on hexatriene, using a polarized double-ζ basis (ANO-L-pVDZ), which gives rise to a Hilbert space containing more than 1038 determinants. These calculations give vertical excitation energies of 5.58 and 5.59 eV, respectively, for the 21Ag and 11Bu states, showing that they are nearly degenerate. The same excitation energies in butadiene/ANO-L-pVDZ were found to be 6.58 and 6.45 eV. In addition to these benchmarks, our calculations strongly support the presence of a previously hypothesiz...
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excited states using semistochastic heat bath Configuration Interaction
Journal of Chemical Physics, 2017Co-Authors: Adam A Holmes, C J Umrigar, Sandeep SharmaAbstract:We extend our recently developed heat-bath Configuration Interaction (HCI) algorithm, and our semistochastic algorithm for performing multireference perturbation theory, to calculate excited-state wavefunctions and energies. We employ time-reversal symmetry, which reduces the memory requirements by more than a factor of two. An extrapolation technique is introduced to reliably extrapolate HCI energies to the full CI limit. The resulting algorithm is used to compute fourteen low-lying potential energy surfaces of the carbon dimer using the cc-pV5Z basis set, with an estimated error in energy of 30-50 μHa compared to full CI. The excitation energies obtained using our algorithm have a mean absolute deviation of 0.02 eV compared to experimental values.
Adam A Holmes - One of the best experts on this subject based on the ideXlab platform.
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fast semistochastic heat bath Configuration Interaction
Journal of Chemical Physics, 2018Co-Authors: Matthew Otten, Adam A Holmes, Sandeep Sharma, C J UmrigarAbstract:This paper presents in detail our fast semistochastic heat-bath Configuration Interaction (SHCI) method for solving the many-body Schrodinger equation. We identify and eliminate computational bottlenecks in both the variational and perturbative steps of the SHCI algorithm. We also describe the parallelization and the key data structures in our implementation, such as the distributed hash table. The improved SHCI algorithm enables us to include in our variational wavefunction two orders of magnitude more determinants than has been reported previously with other selected Configuration Interaction methods. We use our algorithm to calculate an accurate benchmark energy for the chromium dimer with the X2C relativistic Hamiltonian in the cc-pVDZ-DK basis, correlating 28 electrons in 76 spatial orbitals. Our largest calculation uses two billion Slater determinants in the variational space and semistochastically includes perturbative contributions from at least trillions of additional determinants with better than 10-5 Ha statistical uncertainty.
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fast semistochastic heat bath Configuration Interaction
arXiv: Chemical Physics, 2018Co-Authors: Matthew Otten, Adam A Holmes, Sandeep Sharma, C J UmrigarAbstract:This paper presents in detail our fast semistochastic heat-bath Configuration Interaction (SHCI) method for solving the many-body Schrodinger equation. We identify and eliminate computational bottlenecks in both the variational and perturbative steps of the SHCI algorithm. We also describe the parallelization and the key data structures in our implementation, such as the distributed hash table. The improved SHCI algorithm enables us to include in our variational wavefunction two orders of magnitude more determinants than has been reported previously with other selected Configuration Interaction methods. We use our algorithm to calculate an accurate benchmark energy for the chromium dimer with the X2C relativistic Hamiltonian in the cc-pVDZ-DK basis, correlating 28 electrons in 76 spatial orbitals. Our largest calculation uses two billion Slater determinants in the variational space, and semistochastically includes perturbative contributions from at least trillions of additional determinants with better than 10 microhartree statistical uncertainty.
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excited states of methylene polyenes and ozone from heat bath Configuration Interaction
Journal of Physical Chemistry A, 2018Co-Authors: Alan D Chien, C J Umrigar, Matthew Otten, Adam A Holmes, Sandeep Sharma, Paul M ZimmermanAbstract:The electronically excited states of methylene (CH2), ethylene (C2H4), butadiene (C4H6), hexatriene (C6H8), and ozone (O3) have long proven challenging due to their complex mixtures of static and dynamic correlations. The semistochastic heat-bath Configuration Interaction (SHCI) algorithm, which efficiently and systematically approaches the full Configuration Interaction (FCI) limit, is used to provide close approximations to the FCI energies in these systems. This article presents the largest FCI-level calculation to date on hexatriene, using a polarized double-ζ basis (ANO-L-pVDZ), which gives rise to a Hilbert space containing more than 1038 determinants. These calculations give vertical excitation energies of 5.58 and 5.59 eV, respectively, for the 21Ag and 11Bu states, showing that they are nearly degenerate. The same excitation energies in butadiene/ANO-L-pVDZ were found to be 6.58 and 6.45 eV. In addition to these benchmarks, our calculations strongly support the presence of a previously hypothesiz...
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excited states using semistochastic heat bath Configuration Interaction
Journal of Chemical Physics, 2017Co-Authors: Adam A Holmes, C J Umrigar, Sandeep SharmaAbstract:We extend our recently developed heat-bath Configuration Interaction (HCI) algorithm, and our semistochastic algorithm for performing multireference perturbation theory, to calculate excited-state wavefunctions and energies. We employ time-reversal symmetry, which reduces the memory requirements by more than a factor of two. An extrapolation technique is introduced to reliably extrapolate HCI energies to the full CI limit. The resulting algorithm is used to compute fourteen low-lying potential energy surfaces of the carbon dimer using the cc-pV5Z basis set, with an estimated error in energy of 30-50 μHa compared to full CI. The excitation energies obtained using our algorithm have a mean absolute deviation of 0.02 eV compared to experimental values.
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excited states using semistochastic heat bath Configuration Interaction
arXiv: Strongly Correlated Electrons, 2017Co-Authors: Adam A Holmes, C J Umrigar, Sandeep SharmaAbstract:We extend our recently-developed heat-bath Configuration Interaction (HCI) algorithm, and our semistochastic algorithm for performing multireference perturbation theory, to the calculation of excited-state wavefunctions and energies. We employ time-reversal symmetry, which reduces the memory requirements by more than a factor of two. An extrapolation technique is introduced to reliably extrapolate HCI energies to the Full CI limit. The resulting algorithm is used to compute the twelve lowest-lying potential energy surfaces of the carbon dimer using the cc-pV5Z basis set, with an estimated error in energy of 30-50 {\mu}Ha compared to Full CI. The excitation energies obtained using our algorithm have a mean absolute deviation of 0.02 eV compared to experimental values. We also calculate the complete active-space (CAS) energies of the S0, S1, and T0 states of tetracene, which are of relevance to singlet fission, by fully correlating active spaces as large as 18 electrons in 36 orbitals.
Martin Headgordon - One of the best experts on this subject based on the ideXlab platform.
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modern approaches to exact diagonalization and selected Configuration Interaction with the adaptive sampling ci method
Journal of Chemical Theory and Computation, 2020Co-Authors: Norm M Tubman, Martin Headgordon, Daniel C Freeman, Daniel S Levine, Diptarka HaitAbstract:Recent advances in selected CI, including the adaptive sampling Configuration Interaction (ASCI) algorithm and its heat bath extension, have made the ASCI approach competitive with the most accurat...
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non orthogonal Configuration Interaction with single substitutions for the calculation of core excited states
Journal of Chemical Physics, 2018Co-Authors: Katherine J Oosterbaan, Alec F White, Martin HeadgordonAbstract:In this paper, we present the non-orthogonal Configuration Interaction singles (NOCIS) method for calculating core-excited states of closed-shell molecules. NOCIS is a black-box variant of NOCI, which uses A different core-ionized determinants for a molecule with A atoms of a given element to form single substitutions. NOCIS is a variational, spin-pure, size-consistent ab initio method that dramatically improves on standard CIS by capturing essential orbital relaxation effects, in addition to essential Configuration Interaction. We apply it to the calculation of core-excitations for several smaller molecules and demonstrate that it performs competitively with other Hartree-Fock and DFT-based methods. We also benchmark it in several basis sets.
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double spin flip approach within equation of motion coupled cluster and Configuration Interaction formalisms theory implementation and examples
Journal of Chemical Physics, 2009Co-Authors: David Casanova, Lyudmila V Slipchenko, Anna I Krylov, Martin HeadgordonAbstract:The spin-flip (SF) approach is extended to excitations that flip the spin of two electrons to describe multiConfigurational Ms=0 wave functions via high spin quintet references. Equations and implementation of the double SF (2SF) approach within equation-of-motion coupled-cluster (EOM-CC) and Configuration Interaction (CI) formalisms are presented. The numerical performance of the resulting EOM-2SF-CC and 2SF-CI models is demonstrated by calculations of symmetric dissociation of O–H bonds in water, electronic states of linear H4, double CC bond-breaking in ethylene, and low-lying states of trimethylenemethyl diradical and 2,4-didehydrometaxylylene tetraradical. The results of active-space variants of 2SF are very close to the more computationally expensive full-space counterparts. An efficient implementation of the active-space approximation of the 2SF-Configuration Interaction doubles (CID) model termed 2SF-Configuration Interaction singles (CIS) is also reported. The scaling of 2SF-CIS is only N4, which...
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the spin flip extended single excitation Configuration Interaction method
Journal of Chemical Physics, 2008Co-Authors: David Casanova, Martin HeadgordonAbstract:An extension of the spin-flip single excitation Configuration Interaction (SF-CIS) method is introduced. The extension, abbreviated as SF-XCIS, includes all Configurations in which no more than one virtual level of the high spin triplet reference becomes occupied and no more than one doubly occupied level becomes vacant. The number of such Configurations is quadratic with molecule size, and the method is implemented in a direct algorithm whose cost scales in the same way with molecule size as CIS itself, thus permitting applications to large systems. Starting from a spin restricted triplet determinant, SF-XCIS yields spin-pure singlet, triplet, and quintet states, and treats both half-occupied reference orbitals in a fully balanced way to allow application to strongly correlated problems. Tests on bond dissociation in the HF molecule, the torsional potential of ethylene, and excited states of polyenes show encouraging improvements using SF-XCIS compared to SF-CIS and a previously suggested extension, the spin-complete CIS model.
Timothy C Berkelbach - One of the best experts on this subject based on the ideXlab platform.
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vibrational heat bath Configuration Interaction
Journal of Chemical Physics, 2021Co-Authors: Jonathan H Fetherolf, Timothy C BerkelbachAbstract:We introduce vibrational heat-bath Configuration Interaction (VHCI) as an accurate and efficient method for calculating vibrational eigenstates of anharmonic systems. Inspired by its origin in electronic structure theory, VHCI is a selected CI approach that uses a simple criterion to identify important basis states with a pre-sorted list of anharmonic force constants. Screened second-order perturbation theory and simple extrapolation techniques provide significant improvements to variational energy estimates. We benchmark VHCI on four molecules with 12–48 degrees of freedom and use anharmonic potential energy surfaces truncated at fourth and sixth orders. When compared to other methods using the same truncated potentials, VHCI produces vibrational spectra of tens or hundreds of states with sub-wavenumber accuracy at low computational cost.
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vibrational heat bath Configuration Interaction
arXiv: Chemical Physics, 2020Co-Authors: Jonathan H Fetherolf, Timothy C BerkelbachAbstract:We introduce vibrational heat-bath Configuration Interaction (VHCI) as an accurate and efficient method for calculating vibrational eigenstates of anharmonic systems. Inspired by its origin in electronic structure theory, VHCI is a selected CI approach that uses a simple criterion to identify important basis states with a pre-sorted list of anharmonic force constants. Screened second-order perturbation theory and simple extrapolation techniques provide significant improvements to variational energy estimates. We benchmark VHCI on four molecules with 12 to 48 degrees of freedom and use anharmonic potential energy surfaces truncated at fourth and sixth order. For all molecules studied, VHCI produces vibrational spectra of tens or hundreds of states with sub-wavenumber accuracy at low computational cost.
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improved fast randomized iteration approach to full Configuration Interaction
Journal of Chemical Theory and Computation, 2020Co-Authors: Samuel M Greene, Robert J Webber, Jonathan Weare, Timothy C BerkelbachAbstract:We present three modifications to our recently introduced fast randomized iteration method for full Configuration Interaction (FCI-FRI) and investigate their effects on the method's performance for Ne, H2O, and N2. The initiator approximation, originally developed for full Configuration Interaction quantum Monte Carlo, significantly reduces statistical error in FCI-FRI when few samples are used in compression operations, enabling its application to larger chemical systems. The semistochastic extension, which involves exactly preserving a fixed subset of elements in each compression, improves statistical efficiency in some cases but reduces it in others. We also developed a new approach to sampling excitations that yields consistent improvements in statistical efficiency and reductions in computational cost. We discuss possible strategies based on our findings for improving the performance of stochastic quantum chemistry methods more generally.