Analytic Gradients for the Spin-Flip ORMAS-CI Method: Optimizing Minima, Saddle Points, and Conical Intersections

Mato, Joani; Gordon, Mark S.

doi:10.1021/acs.jpca.8b11569

Cited by 13 publications

(13 citation statements)

References 73 publications

(176 reference statements)

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“…144 ORMAS has also been used with spin-flip (SF) in order to solve the problem of spin contamination in SF approaches as will be discussed later. 145,146 Several other variations on CASSCF/MCSCF have been discussed in the literature. Optimizing both the CI coefficients and the orbitals is a computationally intensive problem.…”

Section: Multireference Methodsmentioning

confidence: 99%

“…145,153,338 SF-ORMAS is a single reference CI method that uses a high-spin restricted open shell determinant, where the wave function that has a given multiplicity is generated by an arbitrary amount of spin-flip excitations. 145,146 SF-ORMAS has been developed further with gradients and derivative couplings in order to focus on CoIns. 145,146,343 It has been shown to produce MECI geometries that are similar to MRCI.…”

Section: Topography Of Coinsmentioning

confidence: 99%

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Electronic Structure Methods for the Description of Nonadiabatic Effects and Conical Intersections

Matsika

2021

Chem. Rev.

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Nonadiabatic effects are ubiquitous in photophysics and photochemistry, and therefore, many theoretical developments have been made to properly describe them. Conical intersections are central in nonadiabatic processes, as they promote efficient and ultrafast nonadiabatic transitions between electronic states. A proper theoretical description requires developments in electronic structure and specifically in methods that describe conical intersections between states and nonadiabatic coupling terms. This review focuses on the electronic structure aspects of nonadiabatic processes. We discuss the requirements of electronic structure methods to describe conical intersections and nonadiabatic couplings, how the most common excited state methods perform in describing these effects, and what the recent developments are in expanding the methodology and implementing nonadiabatic couplings.

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Section: Multireference Methodsmentioning

confidence: 99%

Section: Topography Of Coinsmentioning

confidence: 99%

Electronic Structure Methods for the Description of Nonadiabatic Effects and Conical Intersections

Matsika

2021

Chem. Rev.

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“…It is worth mentioning the existence of other electronic structure methods closely related to RAS‐SF. This is the case of the spin‐flip non‐orthogonal CI, 37 the spin‐flip version of the occupation‐restricted multiple active space (SF‐ORMAS), 38,39 which generalizes the orbital partition in RAS‐SF to any number of orbital sets, or quasidegenerate second‐order perturbation approximations 40 . Although these methods are extremely interesting from the fundamental point of view and can potentially provide valuable insight in the characterization of molecular electronic states, they deserve a separate treatment and will not be discussed here.…”

Section: Introductionmentioning

confidence: 99%

Restricted active space configuration interaction methods for strong correlation: Recent developments

Casanova

2021

WIREs Comput Mol Sci

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In this review we outline the theory and recent progress of the restricted active space configuration interaction (RASCI) methodology within the hole and particle approximation. The RASCI is a single reference approach based on the splitting of the orbital space in different subsets, in which the target CI space is expressed by the concomitant number of electrons and empty orbitals in each subspace. Initially, the method was born as a spin complete version of the spin‐flip ansatz able to deal with any number of unpaired electrons. Since then, the method has experienced several improvements related to its theoretical foundations, its efficient implementation, in the characterization of RASCI wave functions, and in the calculation of molecular properties. It has shown to be especially suitable for the characterization of medium to large molecular diradicals and polyradicals, and to the study of photophysical processes involving open‐shell species and/or multiexcitonic states, like in the singlet fission reaction. But, despite this success, several limitations emerge from the sever truncation of the excitation operator, which might translate into inaccurate relative energies between different regions of the potential energy surface, and overestimated (or underestimated) excitation energies. These errors are associated with the lack of dynamic correlation, which has motivated the development and implementation of various improvements based on multi‐reference perturbation theory and to blend the RASCI wave function with density functional theory through range separation of electron–electron interactions. Finally, we discuss some of the properties available from RASCI wave functions and give potential future developments. This article is categorized under: Electronic Structure Theory > Ab Initio Electronic Structure Methods Software > Quantum Chemistry Quantum Computing > Theory Development

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“…pqst pqst (5) where the one-and two-body excitation operators are given as (7) where σ indicates α or β spin, and E ot is the on-top density functional with a dependence on the electronic density ρ and the on-top pair density Π (9) as well as the magnitudes of their gradients…”

Section: Introductionmentioning

confidence: 99%

“…In order to obtain accurate energies, theoretical methods must be employed that are capable of recovering both dynamic and static correlation energy . For this reason, multireference methods are often employed. − In most such approaches, a multiconfiguration self-consistent field (MCSCF) − step is performed first to serve as a reference calculation to recover predominantly static correlation energy, and an additional calculation is performed to recover primarily dynamic correlation energy. Some methods commonly used include complete active space second-order perturbation theory (CASPT2), multireference Møller–Plesset second-order perturbation theory (MRMP2), − or one of their variants and multireference configuration-interaction (MRCI). , One drawback of these methods is that the cost of the post-SCF step often scales poorly with system size, and thus, applications of these methods are often limited to systems of small-to-medium size.…”

Section: Introductionmentioning

confidence: 99%

Calculation of Chemical Reaction Barrier Heights by Multiconfiguration Pair-Density Functional Theory with Correlated Participating Orbitals

et al. 2019

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The accurate description of reaction barrier heights is challenging for quantum mechanical methods due to the need for a balanced treatment of dynamic and static correlation energies because their importance varies during the course of a chemical reaction. While some regions of potential energy surfaces are well-described by a single-reference wave function or by Kohn–Sham density functional theory, in other cases a multireference treatment is needed. For systems with many active electrons, most accurate multireference methods have prohibitive computational scalings with system size. Multiconfiguration pair-density functional theory, MC-PDFT, is a more affordable multireference approach that computes the total electron correlation energy in a single step by using the multiconfiguration kinetic energy, density, and on-top pair density and an on-top density functional. In this work, we apply MC-PDFT to a benchmark database (DBH24/18) of 24 diverse reaction barrier heights. We explore the role of active space and basis set selection on the performance of MC-PDFT. We find that MC-PDFT is able to calculate reaction barrier heights with a similar accuracy to complete active space second order perturbation theory, CASPT2, but at a lower computational cost, and we find that MC-PDFT is less dependent on basis set selection than CASPT2.

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Analytic Gradients for the Spin-Flip ORMAS-CI Method: Optimizing Minima, Saddle Points, and Conical Intersections

Cited by 13 publications

References 73 publications

Electronic Structure Methods for the Description of Nonadiabatic Effects and Conical Intersections

Electronic Structure Methods for the Description of Nonadiabatic Effects and Conical Intersections

Restricted active space configuration interaction methods for strong correlation: Recent developments

Calculation of Chemical Reaction Barrier Heights by Multiconfiguration Pair-Density Functional Theory with Correlated Participating Orbitals

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