Professional Education


  • Bachelor of Science, Wayne State University (2010)
  • Doctor of Philosophy, Michigan State University (2017)

All Publications


  • Efficient Treatment of Large Active Spaces through Multi-GPU Parallel Implementation of Direct Configuration Interaction. Journal of chemical theory and computation Fales, B. S., Martínez, T. J. 2020

    Abstract

    We have extended our graphical processing unit (GPU) accelerated direct configuration interaction program to multiple devices, reducing iteration times for configuration spaces of 165 million determinants to only 3 seconds using NVIDIA P100 GPUs. Similar improvements in the one- and two-particle reduced density matrix (RDM) formation allow for fast analytical energy gradients and electronic properties. Our parallel algorithm enables the calculation of arbitrarily large configuration spaces (limited only by available system memory), with iteration times of 13 minutes for an active space of 18 electrons in 18 orbitals (2.4 billion determinants) using six consumer grade NVIDIA 1080Ti GPUs. These advances enable routine molecular dynamics simulations, geometry optimizations, and absorption spectrum calculations for molecules with large configuration spaces, a task which has heretofore required massive computational effort. In this work, we demonstrate the utility of our program by generating the absorption spectrum for diphenyl acetylene (DPA) at the floating occupation molecular orbital complete active space configuration interaction (FOMO-CASCI) level of theory. Several active spaces were investigated to assess the dependence of spectral features on orbital space dimension.

    View details for DOI 10.1021/acs.jctc.9b01165

    View details for PubMedID 31995369

  • Electronic and Structural Comparisons between Iron(II/III) and Ruthenium(II/III) Imide Analogs INORGANIC CHEMISTRY Aldrich, K. E., Fales, B., Singh, A. K., Staples, R. J., Levine, B. G., McCracken, J., Smith, M. R., Odom, A. L. 2019; 58 (17): 11699–715

    Abstract

    To examine structural and electronic differences between iron and ruthenium imido complexes, a series of compounds was prepared with different phosphine basal sets. The starting material for the ruthenium complexes was Ru(NAr/Ar*)(PMe3)3 (Ru1/Ru1*), where Ar = 2,6-(iPr)2C6H3 and Ar* = 2,4,6-(iPr)3C6H2, which were prepared from cis-RuCl2(PMe3)4 and 2 equiv of LiNHAr/Ar*. The starting materials for the iron complexes were the analogous Fe(NAr/Ar*)(PMe3)3 species (Fe1/Fe1*), which were not isolated but could be generated in situ from FeCl2, PMe3, and LiNHAr/Ar*. With both iron and ruthenium, the PMe3 starting materials underwent phosphine replacement with chelating ligands to give new group 8 imido complexes in the +2 oxidation state. Addition of 1,2-bis(diphenylphosphino)ethane (dppe) to M1/M1* gave Ru(NAr/Ar*)(PMe3)(dppe) and Fe(NAr/Ar*)(PMe3)(dppe). Addition of 1,2-bis(dimethylphosphino)ethane (dmpe) provided Ru(NAr/Ar*)(dmpe)2. A triphos ligand, {P(Me)2CH2}3SitBu (tP3), was also examined. Addition of tP3 to Fe1 provided Fe(NAr)(tP3) (Fe4), but a similar reaction with Ru1 only gave intractable materials. Oxidation of Fe4 with AgSbF6 gave {Fe(NAr)(tP3)}+SbF6- (Fe4a). Oxidation of Ru2 with AgSbF6 gave the unstable cation {Ru(NAr)(PMe3)(dppe)}+, which dimerized in the presence of acetonitrile via C-C bond formation at the aryl group C4 positions, affording {Ru(NAr)(PMe3)(NCMe)(dppe)}2+. This suggested that there was substantial radical character in the imide π system on oxidation and that an aromatic group substituted at the 4-position might provide greater stability. The cations {Fe(NAr*)(PMe3)(dppe)}+ (Fe2a*), {Ru(NAr*)(PMe3)(dppe)}+ (Ru2a*), and Fe4a were examined by EPR spectroscopy, which suggested differences in electronic structure depending on the metal and ligand set. CASPT2 calculations on model systems for Ru2a* and Fe2a* suggested that the large differences in electronic structure are related to the energy gap between the π-antibonding HOMO and the π-bonding HOMO-1. Both the geometry of the phosphines, which is slightly different between the iron and ruthenium analogs, and the metal center seem to contribute to this energetic difference.

    View details for DOI 10.1021/acs.inorgchem.9b01672

    View details for Web of Science ID 000484066500047

    View details for PubMedID 31403782

  • Modeling dynamics of strongly correlated systems with graphics processing unit-accelerated time-dependent multireference methods Levine, B., Peng, W., Fales, B., Durden, A. AMER CHEMICAL SOC. 2019
  • Conical Intersections at the Nanoscale: Molecular Ideas for Materials. Annual review of physical chemistry Levine, B. G., Esch, M. P., Fales, B. S., Hardwick, D. T., Peng, W. T., Shu, Y. 2019

    Abstract

    The ability to predict and describe nonradiative processes in molecules via the identification and characterization of conical intersections is one of the greatest recent successes of theoretical chemistry. Only recently, however, has this concept been extended to materials science, where nonradiative recombination limits the efficiencies of materials for various optoelectronic applications. In this review, we present recent advances in the theoretical study of conical intersections in semiconductor nanomaterials. After briefly introducing conical intersections, we argue that specific defects in materials can induce conical intersections between the ground and first excited electronic states, thus introducing pathways for nonradiative recombination. We present recent developments in theoretical methods, computational tools, and chemical intuition for the prediction of such defect-induced conical intersections. Through examples in various nanomaterials, we illustrate the significance of conical intersections for nanoscience. We also discuss challenges facing research in this area and opportunities for progress. Expected final online publication date for the Annual Review of Physical Chemistry Volume 70 is April 20, 2019. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

    View details for DOI 10.1146/annurev-physchem-042018-052425

    View details for PubMedID 30633637

  • Defect-induced conical intersections in semiconductor nanocrystals Levine, B., Shu, Y., Fales, B., Peng, W., Esch, M., Hardwick, D. AMER CHEMICAL SOC. 2018
  • Dynamics of recombination via conical intersection in a semiconductor nanocrystal CHEMICAL SCIENCE Peng, W., Fales, B., Shu, Y., Levine, B. G. 2018; 9 (3): 681–87

    Abstract

    Conical intersections are well known to introduce nonradiative decay pathways in molecules, but have only recently been implicated in nonradiative recombination processes in materials. Here we apply excited state ab initio molecular dynamics simulations based on a multireference description of the electronic structure to defective silicon nanocrystals up to 1.7 nm in diameter to search for accessible nonradiative recombination pathways. Dangling bond defects are found to induce conical intersections between the ground and first excited electronic states of five systems of various sizes. These defect-induced conical intersections are accessible at energies that are in the visible range (2.4-2.7 eV) and very weakly dependent on particle size. The dynamic simulations suggest that these intersections are accessed 40-60 fs after creation of a defect-localized excitation. This ultrafast recombination is attributed to the fact that Jahn-Teller distortion on the first excited state drives the defect directly towards a conical intersection with the ground electronic state.

    View details for DOI 10.1039/c7sc04221c

    View details for Web of Science ID 000422947000019

    View details for PubMedID 29629136

    View details for PubMedCentralID PMC5869574

  • Large Scale Electron Correlation Calculations: Rank-Reduced Full Configuration Interaction. Journal of chemical theory and computation Fales, B. S., Seritan, S., Settje, N. F., Levine, B. G., Koch, H., Martínez, T. J. 2018

    Abstract

    We present the rank-reduced full configuration interaction (RR-FCI) method, a variational approach for the calculation of extremely large full configuration interaction (FCI) wavefunctions. In this report we show that RR-FCI can provide ground state singlet and triplet energies within kcal/mol accuracy of full CI (FCI) with computational effort scaling as the square root of the number of determinants in the CI space (compared to conventional FCI methods which scale linearly with the number of determinants). Fast graphical processing unit (GPU) accelerated projected σ=Hc matrix-vector product formation enables calculations with configuration spaces as large as 30 electrons in 30 orbitals, corresponding to an FCI calculation with over 2.4x1016 configurations. We apply this method in the context of complete active space configuration interaction calculations to acenes with 2-5 aromatic rings, comparing absolute energies against FCI when possible and singlet/triplet excitation energies against both density matrix renormalization group (DMRG) and experimental results. The dissociation of molecular nitrogen was also examined using both FCI and RR-FCI. In each case we found that RR-FCI provides a low cost alternative to FCI, with particular advantages when relative energies are desired.

    View details for PubMedID 29889519

  • Simulating Electron Dynamics of Complex Molecules with Time-Dependent Complete Active Space Configuration Interaction. Journal of chemical theory and computation Peng, W. T., Fales, B. S., Levine, B. G. 2018

    Abstract

    Time-dependent electronic structure methods are growing in popularity as tools for modeling ultrafast and/or nonlinear processes, for computing spectra, and as the electronic structure component of mean-field molecular dynamics simulations. Time-dependent configuration interaction (TD-CI) offers several advantages over the widely used real-time time-dependent density functional theory, namely that it correctly models Rabi oscillations, it offers a spin-pure description of open-shell systems, and a hierarchy of TD-CI methods can be defined that systematically approach the exact solution of the time-dependent Schrodinger equation (TDSE). In this work we present a novel TD-CI approach that extends TD-CI to large complete active space configuration expansions. Our approach is based on a complete active space TD-CI (TD-CASCI) expansion of the wave function. Extension to large active spaces is enabled by use of a direct configuration interaction approach that eliminates the need to explicitly build, store, or diagonalized the Hamiltonian matrix. Graphics processing unit (GPU) acceleration enables fast solution of the TDSE even for large active spaces-up to 12 electrons in 12 orbitals (853776 determinants) in this work. A symplectic split operator propagator yields long-time norm conservation. We demonstrate the applicability of our approach by computing the response of a large molecule with a strongly correlated ground state, decacene (C42H24), to various pulses (δ-function, transform limited, chirped). Our simulations predict that chirped pulses can be used to induce dipole-forbidden transitions. Simulations of decacene using the 6-31G(d) basis set and a 12 electrons/12 orbitals active space took 20.1 hours to propagate for 100 femtoseconds with a 1 attosecond time step on a single NVIDIA K40 GPU. Convergence with respect to time step is found to depend on the property being computed and the chosen active space.

    View details for DOI 10.1021/acs.jctc.8b00381

    View details for PubMedID 29986143

  • Complete active space configuration interaction from state-averaged configuration interaction singles natural orbitals: Analytic first derivatives and derivative coupling vectors JOURNAL OF CHEMICAL PHYSICS Fales, B., Shu, Y., Levine, B. G., Hohenstein, E. G. 2017; 147 (9): 094104

    Abstract

    A new complete active space configuration interaction (CASCI) method was recently introduced that uses state-averaged natural orbitals from the configuration interaction singles method (configuration interaction singles natural orbital CASCI, CISNO-CASCI). This method has been shown to perform as well or better than state-averaged complete active space self-consistent field for a variety of systems. However, further development and testing of this method have been limited by the lack of available analytic first derivatives of the CISNO-CASCI energy as well as the derivative coupling between electronic states. In the present work, we present a Lagrangian-based formulation of these derivatives as well as a highly efficient implementation of the resulting equations accelerated with graphical processing units. We demonstrate that the CISNO-CASCI method is practical for dynamical simulations of photochemical processes in molecular systems containing hundreds of atoms.

    View details for PubMedID 28886625

  • Understanding Nonradiative Recombination through Defect-Induced Conical Intersections JOURNAL OF PHYSICAL CHEMISTRY LETTERS Shu, Y., Fales, B., Peng, W., Levine, B. G. 2017; 8 (17): 4091–99

    Abstract

    Defects are known to introduce pathways for the nonradiative recombination of electronic excitations in semiconductors, but implicating a specific defect as a nonradiative center remains challenging for both experiment and theory. In this Perspective, we present recent progress toward this goal involving the identification and characterization of defect-induced conical intersections (DICIs), points of degeneracy between the ground and first excited electronic states of semiconductor materials that arise from the deformation of specific defects. Analysis of DICIs does not require the assumption of weak correlation between the electron and hole nor of stationary nuclei. It is demonstrated that in some cases an energetically accessible DICI is present even when no midgap state is predicted by single-particle theories (e.g., density functional theory). We review recent theoretical and computational developments that enable the location of DICIs in semiconductor nanomaterials and present insights into the photoluminescence of silicon nanocrystals gleaned from DICIs.

    View details for DOI 10.1021/acs.jpclett.7b01707

    View details for Web of Science ID 000410600600022

    View details for PubMedID 28799771

  • Robust and Efficient Spin Purification for Determinantal Configuration Interaction JOURNAL OF CHEMICAL THEORY AND COMPUTATION Fales, B., Hohenstein, E. G., Levine, B. G. 2017; 13 (9): 4162–72

    Abstract

    The limited precision of floating point arithmetic can lead to the qualitative and even catastrophic failure of quantum chemical algorithms, especially when high accuracy solutions are sought. For example, numerical errors accumulated while solving for determinantal configuration interaction wave functions via Davidson diagonalization may lead to spin contamination in the trial subspace. This spin contamination may cause the procedure to converge to roots with undesired ⟨Ŝ2⟩, wasting computer time in the best case and leading to incorrect conclusions in the worst. In hopes of finding a suitable remedy, we investigate five purification schemes for ensuring that the eigenvectors have the desired ⟨Ŝ2⟩. These schemes are based on projection, penalty, and iterative approaches. All of these schemes rely on a direct, graphics processing unit-accelerated algorithm for calculating the S2c matrix-vector product. We assess the computational cost and convergence behavior of these methods by application to several benchmark systems and find that the first-order spin penalty method is the optimal choice, though first-order and Löwdin projection approaches also provide fast convergence to the desired spin state. Finally, to demonstrate the utility of these approaches, we computed the lowest several excited states of an open-shell silver cluster (Ag19) using the state-averaged complete active space self-consistent field method, where spin purification was required to ensure spin stability of the CI vector coefficients. Several low-lying states with significant multiply excited character are predicted, suggesting the value of a multireference approach for modeling plasmonic nanomaterials.

    View details for PubMedID 28772070

  • Mechanisms and time-resolved dynamics for trihydrogen cation (H-3(+)) formation from organic molecules in strong laser fields SCIENTIFIC REPORTS Ekanayake, N., Nairat, M., Kaderiya, B., Feizollah, P., Jochim, B., Severt, T., Berry, B., Pandiri, K., Carnes, K. D., Pathak, S., Rolles, D., Rudenko, A., Ben-Itzhak, I., Mancuso, C. A., Fales, B., Jackson, J. E., Levine, B. G., Dantus, M. 2017; 7: 4703

    Abstract

    Strong-field laser-matter interactions often lead to exotic chemical reactions. Trihydrogen cation formation from organic molecules is one such case that requires multiple bonds to break and form. We present evidence for the existence of two different reaction pathways for H3+ formation from organic molecules irradiated by a strong-field laser. Assignment of the two pathways was accomplished through analysis of femtosecond time-resolved strong-field ionization and photoion-photoion coincidence measurements carried out on methanol isotopomers, ethylene glycol, and acetone. Ab initio molecular dynamics simulations suggest the formation occurs via two steps: the initial formation of a neutral hydrogen molecule, followed by the abstraction of a proton from the remaining CHOH2+ fragment by the roaming H2 molecule. This reaction has similarities to the H2 + H2+ mechanism leading to formation of H3+ in the universe. These exotic chemical reaction mechanisms, involving roaming H2 molecules, are found to occur in the ~100 fs timescale. Roaming molecule reactions may help to explain unlikely chemical processes, involving dissociation and formation of multiple chemical bonds, occurring under strong laser fields.

    View details for DOI 10.1038/s41598-017-04666-w

    View details for Web of Science ID 000404846300041

    View details for PubMedID 28680157

    View details for PubMedCentralID PMC5498647

  • A direct-compatible formulation of the coupled perturbed complete active space self-consistent field equations on graphical processing units JOURNAL OF CHEMICAL PHYSICS Snyder, J. W., Fales, B. S., Hohenstein, E. G., Levine, B. G., Martinez, T. J. 2017; 146 (17)

    Abstract

    We recently developed an algorithm to compute response properties for the state-averaged complete active space self-consistent field method (SA-CASSCF) that capitalized on sparsity in the atomic orbital basis. Our original algorithm was limited to treating small to moderate sized active spaces, but the recent development of graphical processing unit (GPU) based direct-configuration interaction algorithms provides an opportunity to extend this to large active spaces. We present here a direct-compatible version of the coupled perturbed equations, enabling us to compute response properties for systems treated with arbitrary active spaces (subject to available memory and computation time). This work demonstrates that the computationally demanding portions of the SA-CASSCF method can be formulated in terms of seven fundamental operations, including Coulomb and exchange matrix builds and their derivatives, as well as, generalized one- and two-particle density matrix and σ vector constructions. As in our previous work, this algorithm exhibits low computational scaling and is accelerated by the use of GPUs, making possible optimizations and nonadiabatic dynamics on systems with O(1000) basis functions and O(100) atoms, respectively.

    View details for DOI 10.1063/1.4979844

    View details for Web of Science ID 000400625800014

    View details for PubMedID 28477593

  • Nanoscale Multireference Quantum Chemistry: Full Configuration Interaction on Graphical Processing Units JOURNAL OF CHEMICAL THEORY AND COMPUTATION Fales, B. S., Levine, B. G. 2015; 11 (10): 4708-4716

    Abstract

    Methods based on a full configuration interaction (FCI) expansion in an active space of orbitals are widely used for modeling chemical phenomena such as bond breaking, multiply excited states, and conical intersections in small-to-medium-sized molecules, but these phenomena occur in systems of all sizes. To scale such calculations up to the nanoscale, we have developed an implementation of FCI in which electron repulsion integral transformation and several of the more expensive steps in σ vector formation are performed on graphical processing unit (GPU) hardware. When applied to a 1.7 × 1.4 × 1.4 nm silicon nanoparticle (Si72H64) described with the polarized, all-electron 6-31G** basis set, our implementation can solve for the ground state of the 16-active-electron/16-active-orbital CASCI Hamiltonian (more than 100,000,000 configurations) in 39 min on a single NVidia K40 GPU.

    View details for DOI 10.1021/acs.jctc.5b00634

    View details for Web of Science ID 000362921700019

    View details for PubMedID 26574260

  • Defect-Induced Conical Intersections Promote Nonradiative Recombination NANO LETTERS Shu, Y., Fales, B. S., Levine, B. G. 2015; 15 (9): 6247-6253

    Abstract

    We apply multireference electronic structure calculations to demonstrate the presence of conical intersections between the ground and the first excited electronic states of three silicon nanocrystals containing defects characteristic of the oxidized silicon surface. These intersections are accessible upon excitation at visible wavelengths and are predicted to facilitate nonradiative recombination with a rate that increases with decreasing particle size. This work illustrates a new framework for identifying defects responsible for nonradiative recombination.

    View details for DOI 10.1021/acs.nanolett.5b02848

    View details for Web of Science ID 000361252700089

    View details for PubMedID 26291975

  • Infrared Multiple Photon Dissociation Action Spectroscopy and Theoretical Studies of Triethyl Phosphate Complexes: Effects of Protonation and Sodium Cationization on Structure JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY Fales, B. S., Fujamade, N. O., Oomens, J., Rodgers, M. T. 2011; 22 (10): 1862-1871

    Abstract

    The gas-phase structures of protonated and sodium cationized complexes of triethyl phosphate, [TEP + H](+) and [TEP + Na](+), are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy using tunable IR radiation generated by a free electron laser, a Fourier transform ion cyclotron resonance mass spectrometer with an electrospray ionization source, and theoretical electronic structure calculations. Measured IRMPD action spectra are compared to linear IR spectra calculated at the B3LYP/6-31 G(d,p) level of theory to identify the structures accessed in the experimental studies. For comparison, theoretical studies of neutral TEP are also performed. Sodium cationization and protonation produce changes in the central phosphate geometry, including an increase in the alkoxy ∠OPO bond angle and shortening of the alkoxy P-O bond. Changes associated with protonation are more pronounced than those produced by sodium cationization.

    View details for DOI 10.1007/s13361-011-0208-7

    View details for Web of Science ID 000295088400019

    View details for PubMedID 21952899

  • Infrared Multiple Photon Dissociation Action Spectroscopy and Theoretical Studies of Diethyl Phosphate Complexes: Effects of Protonation and Sodium Cationization on Structure JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY Fales, B. S., Fujamade, N. O., Nei, Y., Oomens, J., Rodgers, M. T. 2011; 22 (1): 81-92

    Abstract

    The gas-phase structures of deprotonated, protonated, and sodium-cationized complexes of diethyl phosphate (DEP) including [DEP - H](-), [DEP + H](+), [DEP + Na](+), and [DEP - H + 2Na](+) are examined via infrared multiple photon dissociation (IRMPD) action spectroscopy using tunable IR radiation generated by a free electron laser, a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) with an electrospray ionization (ESI) source, and theoretical electronic structure calculations. Measured IRMPD spectra are compared to linear IR spectra calculated at the B3LYP/6-31G(d,p) level of theory to identify the structures accessed in the experimental studies. For comparison, theoretical studies of neutral complexes are also performed. These experiments and calculations suggest that specific geometric changes occur upon the binding of protons and/or sodium cations, including changes correlating to nucleic acid backbone geometry, specifically P-O bond lengths and ∠OPO bond angles. Information from these observations may be used to gain insight into the structures of more complex systems, such as nucleotides and solvated nucleic acids.

    View details for DOI 10.1007/s13361-010-0007-6

    View details for Web of Science ID 000287696300010

    View details for PubMedID 21472547

    View details for PubMedCentralID PMC3042107