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

Abstract

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

Predicting the FCI energy of large systems to chemical accuracy from restricted active space density matrix renormalization group calculations

“…But, remarkably, the optimal ∆ (eq. ( 25), obtained here from nonperturbative considerations) makes the third order perturbation correction E (3) vanish, as is seen from the last formula in (22). The important advance of ( 28)-( 27) over ( 16)-( 17) is that the error is now quantified in powers of ( 20) not (19), which is needed for applicability to realistic systems.…”

“…Quite recently, a cross-fertilization of the conventional restricted active space (RAS) scheme [18][19][20][21][22] with the density matrix renormalization group method [23,24] via the dynamically extended active space procedure [25,26] has emerged as a new powerful method [27] to capture both static and dynamic correlations, and numerical benchmarks on molecules with notorious multi-reference characters have revealed various advantages of the new method with respect to conventional approaches [21,[28][29][30][31].…”

Spin–Orbit Couplings of Open-Shell Systems with Restricted Active Space Configuration Interaction

“…Spatial localization of unpaired electrons has been characterized by the representation of the fractional occupied density (FOD). 44 All calculations have been carried out with a development version of the Q-Chem package. 53…”

Molecular Chirality and Its Monitoring by Ultrafast X-ray Pulses

“…These signals require the computation of core-excited states and their transition multipole moments. The available methods include multi-configurational self-consistent field (MCSCF) approaches such as RASSCF (Restricted Active Space Self Consistent Field), 224,225 density density functional theory such as TDDFT (Time-Dependent Density Function Theory), [226][227][228][229] pertubation theories including the ADC (Algebraic-Diagrammatic Construction methods) schemes and coupled pair theories such as EOM-CC (Equationof-Motion Coupled-Cluster). [230][231][232][233][234] Numerical strategies for optical chiral signals have been extensively discussed by Crawford et al 235 while the computation of core-excited states have recently been reviewed by Norman et al 236 Some specific care must be used to calculate higher order multipoles with acceptable precision.…”