Method development

2024

1. Quantum

   Optimizing Variational Quantum Algorithms with qBang: Efficiently Interweaving Metric and Momentum to Navigate Flat Energy Landscapes

   David Fitzek, Robert S. Jonsson, Werner Dobrautz, and Christian Schäfer

   Quantum, Apr 2024

   arXiv Bib HTML

   @article{Fitzek2024,
     title = {Optimizing Variational Quantum Algorithms with qBang: Efficiently Interweaving Metric and Momentum to Navigate Flat Energy Landscapes},
     volume = {8},
     issn = {2521-327X},
     url = {http://dx.doi.org/10.22331/q-2024-04-09-1313},
     doi = {10.22331/q-2024-04-09-1313},
     journal = {Quantum},
     publisher = {Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften},
     author = {Fitzek, David and Jonsson, Robert S. and Dobrautz, Werner and Sch\"{a}fer, Christian},
     year = {2024},
     month = apr,
     pages = {1313},
     project_devel = {true},
     project_quantum = {true},
     dimensions = {true},
   }

2023

1. JCTC

   The OpenMolcas Web: A Community-Driven Approach to Advancing Computational Chemistry

   Giovanni Li Manni, Ignacio Fdez. Galván, Ali Alavi, Flavia Aleotti, Francesco Aquilante, and 103 more authors

   Journal of Chemical Theory and Computation, May 2023

   Abs Bib HTML ChemRxiv

   The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations.

   @article{LiManni2023,
     doi = {10.1021/acs.jctc.3c00182},
     url = {https://doi.org/10.1021/acs.jctc.3c00182},
     year = {2023},
     month = may,
     publisher = {American Chemical Society ({ACS})},
     author = {Manni, Giovanni Li and Galv{\'{a}}n, Ignacio Fdez. and Alavi, Ali and Aleotti, Flavia and Aquilante, Francesco and Autschbach, Jochen and Avagliano, Davide and Baiardi, Alberto and Bao, Jie J. and Battaglia, Stefano and Birnoschi, Letitia and Blanco-Gonz{\'{a}}lez, Alejandro and Bokarev, Sergey I. and Broer, Ria and Cacciari, Roberto and Calio, Paul B. and Carlson, Rebecca K. and Couto, Rafael Carvalho and Cerd{\'{a}}n, Luis and Chibotaru, Liviu F. and Chilton, Nicholas F. and Church, Jonathan Richard and Conti, Irene and Coriani, Sonia and Cu{\'{e}}llar-Zuquin, Juliana and Daoud, Razan E. and Dattani, Nike and Decleva, Piero and de Graaf, Coen and Delcey, Mickaël G. and Vico, Luca De and Dobrautz, Werner and Dong, Sijia S. and Feng, Rulin and Ferr{\'{e}}, Nicolas and Filatov(Gulak), Michael and Gagliardi, Laura and Garavelli, Marco and Gonz{\'{a}}lez, Leticia and Guan, Yafu and Guo, Meiyuan and Hennefarth, Matthew R. and Hermes, Matthew R. and Hoyer, Chad E. and Huix-Rotllant, Miquel and Jaiswal, Vishal Kumar and Kaiser, Andy and Kaliakin, Danil S. and Khamesian, Marjan and King, Daniel S. and Kochetov, Vladislav and Kro{\'{s}}nicki, Marek and Kumaar, Arpit Arun and Larsson, Ernst D. and Lehtola, Susi and Lepetit, Marie-Bernadette and Lischka, Hans and R{\'{\i}}os, Pablo L{\'{o}}pez and Lundberg, Marcus and Ma, Dongxia and Mai, Sebastian and Marquetand, Philipp and Merritt, Isabella C. D. and Montorsi, Francesco and M\"{o}rchen, Maximilian and Nenov, Artur and Nguyen, Vu Ha Anh and Nishimoto, Yoshio and Oakley, Meagan S. and Olivucci, Massimo and Oppel, Markus and Padula, Daniele and Pandharkar, Riddhish and Phung, Quan Manh and Plasser, Felix and Raggi, Gerardo and Rebolini, Elisa and Reiher, Markus and Rivalta, Ivan and Roca-Sanju{\'{a}}n, Daniel and Romig, Thies and Safari, Arta Anushirwan and S{\'{a}}nchez-Mansilla, Aitor and Sand, Andrew M. and Schapiro, Igor and Scott, Thais R. and Segarra-Mart{\'{\i}}, Javier and Segatta, Francesco and Sergentu, Dumitru-Claudiu and Sharma, Prachi and Shepard, Ron and Shu, Yinan and Staab, Jakob K. and Straatsma, Tjerk P. and S{\o}rensen, Lasse Kragh and Tenorio, Bruno Nunes Cabral and Truhlar, Donald G. and Ungur, Liviu and Vacher, Morgane and Veryazov, Valera and Vo{\ss}, Torben Arne and Weser, Oskar and Wu, Dihua and Yang, Xuchun and Yarkony, David and Zhou, Chen and Zobel, J. Patrick and Lindh, Roland},
     title = {The {OpenMolcas} Web: A Community-Driven Approach to Advancing Computational Chemistry},
     journal = {Journal of Chemical Theory and Computation},
     dimensions = {true},
     project_devel = {true},
     chemrxiv = {https://chemrxiv.org/engage/chemrxiv/article-details/63cab14daadd95af647a2d9e}
   }

2021

1. JCTC

   Spin-Pure Stochastic-CASSCF via GUGA-FCIQMC Applied to Iron–Sulfur Clusters

   Werner Dobrautz, Oskar Weser, Nikolay A. Bogdanov, Ali Alavi, and Giovanni Li Manni

   Journal of Chemical Theory and Computation, Sep 2021

   Abs arXiv Bib HTML

   In this work, we demonstrate how to efficiently compute the one- and two-body reduced density matrices within the spin-adapted full configuration interaction quantum Monte Carlo (FCIQMC) method, which is based on the graphical unitary group approach (GUGA). This allows us to use GUGA-FCIQMC as a spin-pure configuration interaction (CI) eigensolver within the complete active space self-consistent field (CASSCF) procedure and hence to stochastically treat active spaces far larger than conventional CI solvers while variationally relaxing orbitals for specific spin-pure states. We apply the method to investigate the spin ladder in iron–sulfur dimer and tetramer model systems. We demonstrate the importance of the orbital relaxation by comparing the Heisenberg model magnetic coupling parameters from the CASSCF procedure to those from a CI-only (CASCI) procedure based on restricted open-shell Hartree–Fock orbitals. We show that the orbital relaxation differentially stabilizes the lower-spin states, thus enlarging the coupling parameters with respect to the values predicted by ignoring orbital relaxation effects. Moreover, we find that, while CASCI results are well fit by a simple bilinear Heisenberg Hamiltonian, the CASSCF eigenvalues exhibit deviations that necessitate the inclusion of biquadratic terms in the model Hamiltonian.

   @article{Dobrautz2021,
     doi = {10.1021/acs.jctc.1c00589},
     url = {https://doi.org/10.1021/acs.jctc.1c00589},
     year = {2021},
     month = sep,
     publisher = {American Chemical Society ({ACS})},
     volume = {17},
     number = {9},
     pages = {5684--5703},
     author = {Dobrautz, Werner and Weser, Oskar and Bogdanov, Nikolay A. and Alavi, Ali and Manni, Giovanni Li},
     title = {Spin-Pure Stochastic-{CASSCF} via {GUGA}-{FCIQMC} Applied to Iron{\textendash}Sulfur Clusters},
     journal = {Journal of Chemical Theory and Computation},
     dimensions = {true},
     project_guga = {true},
     project_devel = {true},
     project_tm = {true},
     project_fciqmc = {true}
   }

2020

1. JCP

   NECI: N-Electron Configuration Interaction with an emphasis on state-of-the-art stochastic methods

   Kai Guther, Robert J. Anderson, Nick S. Blunt, Nikolay A. Bogdanov, Deidre Cleland, and 20 more authors

   The Journal of Chemical Physics, Jul 2020

   Abs arXiv Bib HTML

   We present NECI, a state-of-the-art implementation of the Full Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm, a method based on a stochastic application of the Hamiltonian matrix on a sparse sampling of the wave function. The program utilizes a very powerful parallelization and scales efficiently to more than 24 000 central processing unit cores. In this paper, we describe the core functionalities of NECI and its recent developments. This includes the capabilities to calculate ground and excited state energies, properties via the one- and two-body reduced density matrices, as well as spectral and Green’s functions for ab initio and model systems. A number of enhancements of the bare FCIQMC algorithm are available within NECI, allowing us to use a partially deterministic formulation of the algorithm, working in a spin-adapted basis or supporting transcorrelated Hamiltonians. NECI supports the FCIDUMP file format for integrals, supplying a convenient interface to numerous quantum chemistry programs, and it is licensed under GPL-3.0.

   @article{Guther2020,
     doi = {10.1063/5.0005754},
     url = {https://doi.org/10.1063/5.0005754},
     year = {2020},
     month = jul,
     publisher = {{AIP} Publishing},
     volume = {153},
     number = {3},
     pages = {034107},
     author = {Guther, Kai and Anderson, Robert J. and Blunt, Nick S. and Bogdanov, Nikolay A. and Cleland, Deidre and Dattani, Nike and Dobrautz, Werner and Ghanem, Khaldoon and Jeszenszki, Peter and Liebermann, Niklas and Manni, Giovanni Li and Lozovoi, Alexander Y. and Luo, Hongjun and Ma, Dongxia and Merz, Florian and Overy, Catherine and Rampp, Markus and Samanta, Pradipta Kumar and Schwarz, Lauretta R. and Shepherd, James J. and Smart, Simon D. and Vitale, Eugenio and Weser, Oskar and Booth, George H. and Alavi, Ali},
     title = {{NECI}: N-Electron Configuration Interaction with an emphasis on state-of-the-art stochastic methods},
     journal = {The Journal of Chemical Physics},
     dimensions = {true},
     project_guga = {true},
     project_devel = {true},
     project_fciqmc = {true}
   }

2019

1. PhD Thesis

   Development of Full Configuration Interaction Quantum Monte Carlo Methods for Strongly Correlated Electron Systems

   Werner Dobrautz

   University of Stuttgart, Mar 2019

   Abs Bib HTML PDF

   Full Configuration Interaction Quantum Monte Carlo (FCIQMC) is a prominent method to calculate the exact solution of the Schrödinger equation in a finite antisymmetric basis and gives access to physical observables through an efficient stochastic sampling of the wavefunction that describes a quantum mechanical system. Although system-agnostic (black-box-like) and numerically exact, its effectiveness depends crucially on the compactness of the wavefunction: a property that gradually decreases as correlation effects become stronger. In this work, we present two -conceptually distinct- approaches to extend the applicability of FCIQMC towards larger and more strongly correlated systems. In the first part, we investigate a spin-adapted formulation of the FCIQMC algorithm, based on the Unitary Group Approach. Exploiting the inherent symmetries of the nonrelativistic molecular Hamiltonian results in a dramatic reduction of the effective Hilbert space size of the problem. The use of a spin-pure basis explicitly resolves the different spin-sectors, even when degenerate, and the absence of spin-contamination ensures the sampled wavefunction is an eigenfunction of the total spin operator. Moreover, targeting specific many-body states with conserved total spin allows an accurate description of chemical processes governed by the intricate interplay of them. We apply the above methodology to obtain results, not otherwise attainable with conventional approaches, for the spin-gap of the high-spin cobalt atom ground- and low-spin excited state and the electron affinity of scandium within chemical accuracy to experiment. Furthermore we establish the ordering of the scandium anion bound states, which has until now not been experimentally determined. In the second part, we investigate a methodology to explicitly incorporate electron correlation into the initial Ansatz of the ground state wavefunction. Such an Ansatz induces a compact description of the wavefunction, which ameliorates the sampling of the configuration space of a system with FCIQMC. Within this approach, we investigate the two-dimensional Hubbard model near half-filling in the intermediate interaction regime, where such an Ansatz can be exactly incorporated by a nonunitary similarity transformation of the Hamiltonian based on a Gutzwiller correlator. This transformation generates novel three-body interactions, tractable due to the stochastic nature of FCIQMC, and leads to a non-Hermitian effective Hamiltonian with extremely compact right eigenvectors. The latter fact allows application of FCIQMC to larger lattice sizes, well beyond the reach of the method applied to the original Hubbard Hamiltonian.

   @phdthesis{dobrautz-phd,
     title = {Development of Full Configuration Interaction Quantum Monte Carlo Methods for Strongly Correlated Electron Systems},
     author = {Dobrautz, Werner},
     school = {University of Stuttgart},
     year = {2019},
     month = mar,
     url = {http://dx.doi.org/10.18419/opus-10593},
     doi = {10.18419/opus-10593},
     project_devel = {true},
     project_fciqmc = {true},
     project_guga = {true},
     project_tc = {true},
     project_thesis = {true},
     owner = {dobrautz},
     timestamp = {2019.04.16}
   }

2. JCP

   Efficient formulation of full configuration interaction quantum Monte Carlo in a spin eigenbasis via the graphical unitary group approach

   Werner Dobrautz, Simon D. Smart, and Ali Alavi

   The Journal of Chemical Physics, Sep 2019

   Abs arXiv Bib HTML

   We provide a spin-adapted formulation of the Full Configuration Interaction Quantum Monte Carlo (FCIQMC) algorithm, based on the Graphical Unitary Group Approach (GUGA), which enables the exploitation of SU(2) symmetry within this stochastic framework. Random excitation generation and matrix element calculation on the Shavitt graph of GUGA can be efficiently implemented via a biasing procedure on the branching diagram. The use of a spin-pure basis explicitly resolves the different spin-sectors and ensures that the stochastically sampled wavefunction is an eigenfunction of the total spin operator Ŝ 2⁠. The method allows for the calculation of states with low or intermediate spin in systems dominated by Hund’s first rule, which are otherwise generally inaccessible. Furthermore, in systems with small spin gaps, the new methodology enables much more rapid convergence with respect to walker number and simulation time. Some illustrative applications of the GUGA-FCIQMC method are provided: computation of the 2F − 4F spin gap of the cobalt atom in large basis sets, achieving chemical accuracy to experiment, and the Σg+1⁠, Σg+3⁠, Σg+5⁠, and Σg+7 spin-gaps of the stretched N2 molecule, an archetypal strongly correlated system.

   @article{Dobrautz2019,
     doi = {10.1063/1.5108908},
     url = {https://doi.org/10.1063/1.5108908},
     year = {2019},
     month = sep,
     publisher = {{AIP} Publishing},
     volume = {151},
     number = {9},
     author = {Dobrautz, Werner and Smart, Simon D. and Alavi, Ali},
     title = {Efficient formulation of full configuration interaction quantum Monte Carlo in a spin eigenbasis via the graphical unitary group approach},
     journal = {The Journal of Chemical Physics},
     dimensions = {true},
     project_guga = {true},
     project_devel = {true},
     project_fciqmc = {true}
   }

2018

1. PRL

   Time Propagation and Spectroscopy of Fermionic Systems Using a Stochastic Technique

   Kai Guther, Werner Dobrautz, Olle Gunnarsson, and Ali Alavi

   Phys. Rev. Lett., Aug 2018

   Abs arXiv Bib HTML

   We present a stochastic method for solving the time-dependent Schrödinger equation, generalizing a ground state full configuration interaction quantum Monte Carlo method. By performing the time integration in the complex plane close to the real-time axis, the numerical effort is kept manageable and the analytic continuation to real frequencies is efficient. This allows us to perform ab initio calculation of electron spectra for strongly correlated systems. The method can be used as a cluster solver for embedding schemes.

   @article{PhysRevLett.121.056401,
     title = {Time Propagation and Spectroscopy of Fermionic Systems Using a Stochastic Technique},
     author = {Guther, Kai and Dobrautz, Werner and Gunnarsson, Olle and Alavi, Ali},
     journal = {Phys. Rev. Lett.},
     volume = {121},
     issue = {5},
     pages = {056401},
     numpages = {5},
     year = {2018},
     month = aug,
     publisher = {American Physical Society},
     doi = {10.1103/PhysRevLett.121.056401},
     url = {https://link.aps.org/doi/10.1103/PhysRevLett.121.056401},
     dimensions = {true},
     project_devel = {true},
     project_fciqmc = {true}
   }