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Laura Gagliardi - One of the best experts on this subject based on the ideXlab platform.
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multiconfigurational Self Consistent Field theory with density matrix embedding the localized active space Self Consistent Field method
Journal of Chemical Theory and Computation, 2019Co-Authors: Matthew R Hermes, Laura GagliardiAbstract:Density matrix embedding theory (DMET) is a fully quantum-mechanical embedding method which shows great promise as a method of defeating the inherent exponential cost scaling of multiconfigurational wave function-based calculations by breaking large systems into smaller, coupled subsystems. However, we recently [ Pham et al. J. Chem. Theory Comput. 2018 , 14 , 1960 .] encountered evidence that the approximate single-determinantal bath picture inherent to DMET is sometimes problematic when the complete active space Self-Consistent Field (CASSCF) is used as a solver and the method is applied to realistic models of strongly correlated molecules. Here, we show this problem can be defeated by generalizing DMET to use a multiconfigurational wave function as a bath without sacrificing practically attractive features of DMET, such as a second-quantization form of the embedded subsystem Hamiltonian, by dividing the active space into unentangled active subspaces each localized to one fragment. We introduce the term localized active space (LAS) to refer to this kind of wave function. The LAS bath wave function can be obtained by the DMET algorithm itSelf in a Self-Consistent manner, and we refer to this approach, introduced here for the first time, as the localized active space Self-Consistent Field (LASSCF) method. LASSCF exploits a modified DMET algorithm, but it is a variational wave function method; it does not require DMET's ambiguous error function minimization, and it reproduces full-molecule CASSCF in cases where comparable DMET calculations fail. Our results for test calculations on the nitrogen double-bond dissociation potential energy curves of several diazene molecules suggest that LASSCF can be an appropriate starting point for a perturbative treatment. Outside of the context of embedding, the LAS wave function is inherently an attractive alternative to a CAS wave function because of its favorable cost scaling, which is exponential only with respect to the size of individual fragment active subspaces, rather than the whole active space of the entire system.
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second order perturbation theory for generalized active space Self Consistent Field wave functions
Journal of Chemical Theory and Computation, 2016Co-Authors: Giovanni Li Manni, Jeppe Olsen, Laura GagliardiAbstract:A multireference second-order perturbation theory approach based on the generalized active space Self-Consistent-Field (GASSCF) wave function is presented. Compared with the complete active space (CAS) and restricted active space (RAS) wave functions, GAS wave functions are more flexible and can employ larger active spaces and/or different truncations of the configuration interaction expansion. With GASSCF, one can explore chemical systems that are not affordable with either CASSCF or RASSCF. Perturbation theory to second order on top of GAS wave functions (GASPT2) has been implemented to recover the remaining electron correlation. The method has been benchmarked by computing the chromium dimer ground-state potential energy curve. These calculations show that GASPT2 gives results similar to CASPT2 even with a configuration interaction expansion much smaller than the corresponding CAS expansion.
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the generalized active space concept in multiconfigurational Self Consistent Field methods
Journal of Chemical Physics, 2011Co-Authors: Giovanni Li Manni, Laura GagliardiAbstract:A multiconfigurational Self-Consistent Field method based on the concept of generalized active space (GAS) is presented. GAS wave functions are obtained by defining an arbitrary number of active spaces with arbitrary occupation constraints. By a suitable choice of the GAS spaces, numerous ineffective configurations present in a large complete active space (CAS) can be removed, while keeping the important ones in the CI space. As a consequence, the GAS Self-Consistent Field approach retains the accuracy of the CAS Self-Consistent Field (CASSCF) ansatz and, at the same time, can deal with larger active spaces, which would be unaffordable at the CASSCF level. Test calculations on the Gd atom, Gd2 molecule, and oxoMn(salen) complex are presented. They show that GAS wave functions achieve the same accuracy as CAS wave functions on systems that would be prohibitive at the CAS level.
Bjorn O Roos - One of the best experts on this subject based on the ideXlab platform.
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accurate ab initio density fitting for multiconfigurational Self Consistent Field methods
Journal of Chemical Physics, 2008Co-Authors: Francesco Aquilante, Bjorn O Roos, Roland Lindh, Thomas Bondo Pedersen, Alfredo Sanchez De Meras, Henrik KochAbstract:Using Cholesky decomposition and density fitting to approximate the electron repulsion integrals, an implementation of the complete active space Self-Consistent Field (CASSCF) method suitable for large-scale applications is presented. Sample calculations on benzene, diaquo-tetra-mu-acetato-dicopper(II), and diuraniumendofullerene demonstrate that the Cholesky and density fitting approximations allow larger basis sets and larger systems to be treated at the CASSCF level of theory with controllable accuracy. While strict error control is an inherent property of the Cholesky approximation, errors arising from the density fitting approach are managed by using a recently proposed class of auxiliary basis sets constructed from Cholesky decomposition of the atomic electron repulsion integrals.
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second order perturbation theory with a complete active space Self Consistent Field reference function
Journal of Chemical Physics, 1992Co-Authors: Kerstin Andersson, Perake Malmqvist, Bjorn O RoosAbstract:The recently implemented second‐order perturbation theory based on a complete active space Self‐Consistent Field reference function has been extended by allowing the Fock‐type one‐electron operator, which defines the zeroth‐order Hamiltonian to have nonzero elements also in nondiagonal matrix blocks. The computer implementation is now less straightforward and more computer time will be needed in obtaining the second‐order energy. The method is illustrated in a series of calculations on N2, NO, O2, CH3, CH2, and F−.
James R Chelikowsky - One of the best experts on this subject based on the ideXlab platform.
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Parallel Self-Consistent-Field calculations via Chebyshev-filtered subspace acceleration.
Physical review. E Statistical nonlinear and soft matter physics, 2006Co-Authors: Yunkai Zhou, Yousef Saad, Murilo L Tiago, James R ChelikowskyAbstract:Solving the Kohn-Sham eigenvalue problem constitutes the most computationally expensive part in Self-Consistent density functional theory (DFT) calculations. In a previous paper, we have proposed a nonlinear Chebyshev-filtered subspace iteration method, which avoids computing explicit eigenvectors except at the first Self-Consistent-Field (SCF) iteration. The method may be viewed as an approach to solve the original nonlinear Kohn-Sham equation by a nonlinear subspace iteration technique, without emphasizing the intermediate linearized Kohn-Sham eigenvalue problems. It reaches Self-consistency within a similar number of SCF iterations as eigensolver-based approaches. However, replacing the standard diagonalization at each SCF iteration by a Chebyshev subspace filtering step results in a significant speedup over methods based on standard diagonalization. Here, we discuss an approach for implementing this method in multi-processor, parallel environment. Numerical results are presented to show that the method enables to perform a class of highly challenging DFT calculations that were not feasible before.
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Self Consistent Field calculations using chebyshev filtered subspace iteration
Journal of Computational Physics, 2006Co-Authors: Yunkai Zhou, Yousef Saad, Murilo L Tiago, James R ChelikowskyAbstract:The power of density functional theory is often limited by the high computational demand in solving an eigenvalue problem at each Self-Consistent-Field (SCF) iteration. The method presented in this paper replaces the explicit eigenvalue calculations by an approximation of the wanted invariant subspace, obtained with the help of well-selected Chebyshev polynomial filters. In this approach, only the initial SCF iteration requires solving an eigenvalue problem, in order to provide a good initial subspace. In the remaining SCF iterations, no iterative eigensolvers are involved. Instead, Chebyshev polynomials are used to refine the subspace. The subspace iteration at each step is easily five to ten times faster than solving a corresponding eigenproblem by the most efficient eigen-algorithms. Moreover, the subspace iteration reaches Self-consistency within roughly the same number of steps as an eigensolver-based approach. This results in a significantly faster SCF iteration.
Benedetta Mennucci - One of the best experts on this subject based on the ideXlab platform.
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polarizable qm mm multiconfiguration Self Consistent Field approach with state specific corrections environment effects on cytosine absorption spectrum
Journal of Chemical Theory and Computation, 2015Co-Authors: Benedetta Mennucci, Michael A Robb, Lluis Blancafort, Carles CurutchetAbstract:We present the formulation and implementation of a polarizable quantum mechanics/molecular mechanics (QM/MM) strategy to describe environment effects in multiconfiguration Self-Consistent Field calculations. The strategy is applied to the calculation of the vertical absorption spectrum of cytosine in water. In our approach, mutual polarization of the solute and the solvent is solved Self-Consistently at the complete-active-space Self-Consistent-Field (CASSCF) level, and the resulting set of charges and dipoles is used to calculate vertical excitation energies using the complete-active-space second-order perturbative (CASPT2) approach and its multistate (MS-CASPT2) variant. In order to treat multiple excited states, we converge the solvent polarization with respect to the state-averaged density of the solute. In order to obtain the final energies, however, we introduce a state-specific correction, where the solvent polarization is recomputed with the density of each state, and demonstrate that this correct...
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excited states and solvatochromic shifts within a nonequilibrium solvation approach a new formulation of the integral equation formalism method at the Self Consistent Field configuration interaction and multiconfiguration Self Consistent Field level
Journal of Chemical Physics, 1998Co-Authors: Benedetta Mennucci, Roberto Cammi, Jacopo TomasiAbstract:The effects of the solvation on excited states are studied in the framework of a nonequilibrium regime between solute and solvent charge distributions. The approach, which exploits a separation of the polarization into slow and fast components, is inserted in a new formulation of the recently developed continuum solvation model known as integral equation formalism. This new version, implying a large computational gain both in time consuming and memory occupation, is here implemented at the Hartree–Fock level as well as at the multiconfiguration Self-Consistent Field and configuration interaction levels. Examples of application of the method to solvatochromic shifts for low-lying excitation energies of formaldehyde, acetaldehyde, and acetone in water are shown.
Eric Neuscamman - One of the best experts on this subject based on the ideXlab platform.
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a Self Consistent Field formulation of excited state mean Field theory
Journal of Chemical Physics, 2020Co-Authors: Tarini S Hardikar, Eric NeuscammanAbstract:We show that, as in Hartree-Fock theory, the orbitals for excited state mean Field theory can be optimized via a Self-Consistent one-electron equation in which electron-electron repulsion is accounted for through mean Field operators. In addition to showing that this excited state ansatz is sufficiently close to a mean Field product state to admit a one-electron formulation, this approach brings the orbital optimization speed to within roughly a factor of two of ground state mean Field theory. The approach parallels Hartree Fock theory in multiple ways, including the presence of a commutator condition, a one-electron mean-Field working equation, and acceleration via direct inversion in the iterative subspace. When combined with a configuration interaction singles Davidson solver for the excitation coefficients, the Self-Consistent Field formulation dramatically reduces the cost of the theory compared to previous approaches based on quasi-Newton descent.